Fix problems in ns port found by static checking.

Fix problems in ns port found by static checking.

* nsterm.m: Include <pthread.h>, for pthread_mutex_lock etc.
(hold_event, setPosition:portion:whole:): Send SIGIO only to self,
not to process group.
(ns_select): Use emacs_write, not write, as that's more robust
in the presence of signals.
(fd_handler:): Check for read errors.