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/gets.c

@@ -32,6 +32,7 @@
  */
 
 #include <stdio.h>
+#include "local.h"
 
 __warn_references(gets,
     "warning: gets() is very unsafe; consider using fgets()");
@@ -42,14 +43,17 @@ gets(char *buf)
 	int c;
 	char *s;
 
-	for (s = buf; (c = getchar()) != '\n';)
+	FLOCKFILE(stdin);
+	for (s = buf; (c = getchar_unlocked()) != '\n';)
 		if (c == EOF)
-			if (s == buf)
+			if (s == buf) {
+				FUNLOCKFILE(stdin);
 				return (NULL);
-			else
+			} else
 				break;
 		else
 			*s++ = c;
 	*s = '\0';
+	FUNLOCKFILE(stdin);
 	return (buf);
 }