split internal lock API out of libc.h, creating lock.h

this further reduces the number of source files which need to include
libc.h and thereby be potentially exposed to libc global state and
internals.

this will also facilitate further improvements like adding an inline
fast-path, if we want to do so later.

src/misc/syslog.c

@@ -9,7 +9,7 @@
 #include <pthread.h>
 #include <errno.h>
 #include <fcntl.h>
-#include "libc.h"
+#include "lock.h"
 
 static volatile int lock[1];
 static char log_ident[32];