Split p_strlen into its own header

We need this from util.h and posix.h, but the latter includes common.h
which includes util.h, which means p_strlen is not defined by the time
we get to git__strndup().

Split the definition on p_strlen() off into its own header so we can use
it in util.h.

src/posix.h

@@ -89,18 +89,7 @@ extern struct tm * p_gmtime_r (const time_t *timer, struct tm *result);
 #	include "unix/posix.h"
 #endif
 
-#if defined(__MINGW32__) || defined(__sun) || defined(__APPLE__)
-#   define NO_STRNLEN
-#endif
-
-#ifdef NO_STRNLEN
-GIT_INLINE(size_t) p_strnlen(const char *s, size_t maxlen) {
-	const char *end = memchr(s, 0, maxlen);
-	return end ? (size_t)(end - s) : maxlen;
-}
-#else
-#   define p_strnlen strnlen
-#endif
+#include "strnlen.h"
 
 #ifdef NO_READDIR_R
 #	include <dirent.h>

src/strnlen.h

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_strlen_h__
+#define INCLUDE_strlen_h__
+
+#if defined(__MINGW32__) || defined(__sun) || defined(__APPLE__)
+#   define NO_STRNLEN
+#endif
+
+#ifdef NO_STRNLEN
+GIT_INLINE(size_t) p_strnlen(const char *s, size_t maxlen) {
+	const char *end = memchr(s, 0, maxlen);
+	return end ? (size_t)(end - s) : maxlen;
+}
+#else
+#   define p_strnlen strnlen
+#endif
+
+#endif

src/util.h

@@ -7,8 +7,8 @@
 #ifndef INCLUDE_util_h__
 #define INCLUDE_util_h__
 
-#include "posix.h"
 #include "common.h"
+#include "strnlen.h"
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 #define bitsizeof(x) (CHAR_BIT * sizeof(x))