NetBSD: Implement OS_Unix::get_executable_path()

Same implementation as OpenBSD seems to work fine.

(cherry picked from commit 01185acecbdfc95b1b9421dac32ee64705108a4b)

drivers/unix/os_unix.cpp

@@ -48,7 +48,7 @@
 #include <mach/mach_time.h>
 #endif
 
-#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
 #include <sys/param.h>
 #include <sys/sysctl.h>
 #endif
@@ -509,7 +509,7 @@ String OS_Unix::get_executable_path() const {
 		return OS::get_executable_path();
 	}
 	return b;
-#elif defined(__OpenBSD__)
+#elif defined(__OpenBSD__) || defined(__NetBSD__)
 	char resolved_path[MAXPATHLEN];
 
 	realpath(OS::get_executable_path().utf8().get_data(), resolved_path);