apps: getpid should return process id not thread id

Summary:
   following the change in the nuttx kernel, implement the right semantics:
1. getpid should return the main thread id
2. gettid should return the current thread id

Refer:
 https://github.com/apache/incubator-nuttx/issues/2499
 https://github.com/apache/incubator-nuttx/pull/2518

Nuttx Kernel PR:
 https://github.com/apache/incubator-nuttx/pull/7597

update apps code

Testing PASSED with qemu( 32/64 )

Signed-off-by: qinwei1 <qinwei1@xiaomi.com>

netutils/netlib/netlib_getnbtab.c

@@ -123,7 +123,7 @@ ssize_t netlib_get_nbtable(FAR struct neighbor_entry_s *nbtab,
 
   /* Bind the socket so that we can use send() and receive() */
 
-  pid            = getpid();
+  pid            = gettid();
   addr.nl_family = AF_NETLINK;
   addr.nl_pad    = 0;
   addr.nl_pid    = pid;