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
diff --git a/libc/stdio/putc.c b/libc/stdio/putc.c
index 9250215..2b05504 100644
--- a/libc/stdio/putc.c
+++ b/libc/stdio/putc.c
@@ -60,8 +60,8 @@
 {
 	int ret;
 
-	flockfile(fp);
+	FLOCKFILE(fp);
 	ret = putc_unlocked(c, fp);
-	funlockfile(fp);
+	FUNLOCKFILE(fp);
 	return (ret);
 }