utils: Add memrchr(3) replacement for platforms that don't support it

For instance, on Mac OS X memrchr(3) is not provided by the C library.

src/libstrongswan/utils/utils.c

@@ -103,7 +103,7 @@ void memwipe_noinline(void *ptr, size_t n)
  */
 void *memstr(const void *haystack, const char *needle, size_t n)
 {
-	unsigned const char *pos = haystack;
+	const u_char *pos = haystack;
 	size_t l;
 
 	if (!haystack || !needle || (l = strlen(needle)) == 0)
@@ -120,6 +120,28 @@ void *memstr(const void *haystack, const char *needle, size_t n)
 	return NULL;
 }
 
+/**
+ * Described in header.
+ */
+void *utils_memrchr(const void *s, int c, size_t n)
+{
+	const u_char *pos;
+
+	if (!s || !n)
+	{
+		return NULL;
+	}
+
+	for (pos = s + n - 1; pos >= (u_char*)s; pos--)
+	{
+		if (*pos == (u_char)c)
+		{
+			return (void*)pos;
+		}
+	}
+	return NULL;
+}
+
 /**
  * Described in header.
  */

src/libstrongswan/utils/utils.h

@@ -464,6 +464,20 @@ static inline void memwipe(void *ptr, size_t n)
  */
 void *memstr(const void *haystack, const char *needle, size_t n);
 
+/**
+ * Replacement for memrchr(3) if it is not provided by the C library.
+ *
+ * @param s		start of the memory area to search
+ * @param c		character to search
+ * @param n		length of memory area to search
+ * @return		pointer to the found character or NULL
+ */
+void *utils_memrchr(const void *s, int c, size_t n);
+
+#ifndef HAVE_MEMRCHR
+#define memrchr(s,c,n) utils_memrchr(s,c,n)
+#endif
+
 /**
  * Translates the characters in the given string, searching for characters
  * in 'from' and mapping them to characters in 'to'.