fix build on freebsd

Add some needed includes
Provide freebsd variation of get_executable_path
Provide variation of execv so that either full path
or appname to be found within $PATH can be used

drivers/unix/ip_unix.cpp

@@ -50,11 +50,16 @@
  #ifdef ANDROID_ENABLED
   #include "platform/android/ifaddrs_android.h"
  #else
+  #ifdef __FreeBSD__
+   #include <sys/types.h>
+  #endif
   #include <ifaddrs.h>
  #endif
  #include <arpa/inet.h>
  #include <sys/socket.h>
-
+ #ifdef __FreeBSD__
+  #include <netinet/in.h>
+ #endif
 #endif
 
 IP_Address IP_Unix::_resolve_hostname(const String& p_hostname) {