Detect accept4() on specific versions of various platforms (#294)

This PR has mainly done three things:

1. Enable `accept4()` on DragonFlyBSD
2. Fix the failures of determining the presence of `accept4()` due to
   the missing <sys/param.h> on two OSs: NetBSD, OpenBSD
3. Drop the support of FreeBSD < 10.0 for `valkey`

### References
- [param.h in
DragonFlyBSD](7485684fa5/sys/sys/param.h (L129-L257))
- [param.h in
FreeBSD](https://github.com/freebsd/freebsd-src/blob/main/sys/sys/param.h#L46-L76)
- [param.h in
NetBSD](b5f8d2f930/sys/sys/param.h (L53-L70))
- [param.h in
OpenBSD](d9c286e032/sys/sys/param.h (L40-L45))

---------

Signed-off-by: Andy Pan <i@andypan.me>

src/config.h

@@ -30,6 +30,8 @@
 #ifndef CONFIG_H
 #define CONFIG_H
 
+#include <sys/param.h>
+
 #ifdef __APPLE__
 #include <fcntl.h> // for fcntl(fd, F_FULLFSYNC)
 #include <AvailabilityMacros.h>
@@ -96,9 +98,11 @@
 #endif
 
 /* Test for accept4() */
-#if defined(__linux__) || defined(OpenBSD5_7) || \
-    (__FreeBSD__ >= 10 || __FreeBSD_version >= 1000000) || \
-    (defined(NetBSD8_0) || __NetBSD_Version__ >= 800000000)
+#if defined(__linux__) || \
+    defined(__FreeBSD__) || \
+    defined(OpenBSD5_7) || \
+    (defined(__DragonFly__) && __DragonFly_version >= 400305) || \
+    (defined(__NetBSD__) && (defined(NetBSD8_0) || __NetBSD_Version__ >= 800000000))
 #define HAVE_ACCEPT4 1
 #endif
 
@@ -316,7 +320,7 @@ void setcpuaffinity(const char *cpulist);
 #endif
 
 /* Test for posix_fadvise() */
-#if defined(__linux__) || __FreeBSD__ >= 10
+#if defined(__linux__) || defined(__FreeBSD__)
 #define HAVE_FADVISE
 #endif