Fix the handle locking in stdio

Fix the handle locking in stdio to use flockfile/funlockfile
internally when and where required.  Macros in <stdio.h> are updated
to automatically call the underlying functions when the process is
threaded to obtain the necessary locking.  A private mutex is added
to protect __sglue, the internal list of FILE handles, and another
to protect the one-time initialization.  Some routines in libc that
use getc() change to use getc_unlocked() as they're either protected
by their own lock or aren't thread-safe routines anyway.

Based on OpenBSD change by guenther@openbsd.org
http://www.mail-archive.com/source-changes@cvs.openbsd.org/msg01015.html

Bug: 3446659
Change-Id: Ie82116e358c541718d6709ec45ca6796be5a007b

libc/stdio/snprintf.c

@@ -60,7 +60,7 @@ snprintf(char *str, size_t n, const char *fmt, ...)
 	f._bf._base = f._p = (unsigned char *)str;
 	f._bf._size = f._w = n - 1;
 	va_start(ap, fmt);
-	ret = vfprintf(&f, fmt, ap);
+	ret = __vfprintf(&f, fmt, ap);
 	va_end(ap);
 	*f._p = '\0';
 	return (ret);