Add pg_strnlen() a portable implementation of strlen.

As the OS version is likely going to be more optimized, fall back to
it if available, as detected by configure.

src/common/string.c

@@ -41,3 +41,23 @@ pg_str_endswith(const char *str, const char *end)
 	str += slen - elen;
 	return strcmp(str, end) == 0;
 }
+
+
+/*
+ * Portable version of posix' strnlen.
+ *
+ * Returns the number of characters before a null-byte in the string pointed
+ * to by str, unless there's no null-byte before maxlen. In the latter case
+ * maxlen is returned.
+ */
+#ifndef HAVE_STRNLEN
+size_t
+pg_strnlen(const char *str, size_t maxlen)
+{
+	const char *p = str;
+
+	while (maxlen-- > 0 && *p)
+		p++;
+	return p - str;
+}
+#endif