Re-indent code and reduce code being complied on Solaris for anetKeepAlive (#12914)

This is a follow-up PR for #12782, in which we introduced nested
preprocessor directives for TCP keep-alive on Solaris and added
redundant indentation for code. Besides, it could result in unreachable
code due to the lack of `#else` on the latest Solaris 11.4 where
`TCP_KEEPIDLE`, `TCP_KEEPINTVL`, and `TCP_KEEPCNT` are available. As a
result, this PR does three main things:

- To eliminate the redundant indention for C code in nested preprocessor
directives
- To add `#else` directives and move `TCP_KEEPALIVE_THRESHOLD` +
`TCP_KEEPALIVE_ABORT_THRESHOLD` settings under it, avoid unreachable
code and compiler warnings when `#if defined(TCP_KEEPIDLE) &&
defined(TCP_KEEPINTVL) && defined(TCP_KEEPCNT)` is met on Solaris 11.4
- To remove a few trailing whitespace in comments

src/anet.c

@@ -145,7 +145,7 @@ int anetKeepAlive(char *err, int fd, int interval)
     int intvl;
     int cnt;
 
-/* There are platforms that are expected to support the full mechanism of TCP keep-alive,
+    /* There are platforms that are expected to support the full mechanism of TCP keep-alive,
      * we want the compiler to emit warnings of unused variables if the preprocessor directives
      * somehow fail, and other than those platforms, just omit these warnings if they happen.
      */
@@ -158,11 +158,12 @@ int anetKeepAlive(char *err, int fd, int interval)
     UNUSED(cnt);
 #endif
 
-/* The implementation of TCP keep-alive on Solaris/SmartOS is a bit unusual 
- * compared to other Unix-like systems. 
- * Thus, we need to specialize it on Solaris. */
 #ifdef __sun
-    /* There are two keep-alive mechanisms on Solaris:
+    /* The implementation of TCP keep-alive on Solaris/SmartOS is a bit unusual
+     * compared to other Unix-like systems.
+     * Thus, we need to specialize it on Solaris.
+     *
+     * There are two keep-alive mechanisms on Solaris:
      * - By default, the first keep-alive probe is sent out after a TCP connection is idle for two hours.
      * If the peer does not respond to the probe within eight minutes, the TCP connection is aborted.
      * You can alter the interval for sending out the first probe using the socket option TCP_KEEPALIVE_THRESHOLD
@@ -173,7 +174,7 @@ int anetKeepAlive(char *err, int fd, int interval)
      * The option value is an unsigned integer in milliseconds. The value zero indicates that TCP should never time out and
      * abort the connection when probing. The system default is controlled by the TCP ndd parameter tcp_keepalive_abort_interval.
      * The default is eight minutes.
-
+     *
      * - The second implementation is activated if socket option TCP_KEEPINTVL and/or TCP_KEEPCNT are set.
      * The time between each consequent probes is set by TCP_KEEPINTVL in seconds.
      * The minimum value is ten seconds. The maximum is ten days, while the default is two hours.
@@ -186,24 +187,24 @@ int anetKeepAlive(char *err, int fd, int interval)
 
     /* `TCP_KEEPIDLE`, `TCP_KEEPINTVL`, and `TCP_KEEPCNT` were not available on Solaris
      * until version 11.4, but let's take a chance here. */
-    #if defined(TCP_KEEPIDLE) && defined(TCP_KEEPINTVL) && defined(TCP_KEEPCNT)
+#if defined(TCP_KEEPIDLE) && defined(TCP_KEEPINTVL) && defined(TCP_KEEPCNT)
     if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle))) {
         anetSetError(err, "setsockopt TCP_KEEPIDLE: %s\n", strerror(errno));
         return ANET_ERR;
     }
+
     intvl = idle/3;
     if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPINTVL, &intvl, sizeof(intvl))) {
         anetSetError(err, "setsockopt TCP_KEEPINTVL: %s\n", strerror(errno));
         return ANET_ERR;
     }
+
     cnt = 3;
     if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPCNT, &cnt, sizeof(cnt))) {
         anetSetError(err, "setsockopt TCP_KEEPCNT: %s\n", strerror(errno));
         return ANET_ERR;
     }
-        return ANET_OK;
+#else
-    #endif
-
     /* Fall back to the first implementation of tcp-alive mechanism for older Solaris,
      * simulate the tcp-alive mechanism on other platforms via `TCP_KEEPALIVE_THRESHOLD` + `TCP_KEEPALIVE_ABORT_THRESHOLD`.
      */
@@ -222,8 +223,10 @@ int anetKeepAlive(char *err, int fd, int interval)
         anetSetError(err, "setsockopt TCP_KEEPCNT: %s\n", strerror(errno));
         return ANET_ERR;
     }
+#endif
 
     return ANET_OK;
+
 #endif
 
 #ifdef TCP_KEEPIDLE