implement memrchr (nonstandard) and optimize strrchr in terms of it

include/string.h

@@ -74,6 +74,7 @@ int strncasecmp (const char *, const char *, size_t);
 char *strchrnul(const char *, int);
 char *strcasestr(const char *, const char *);
 char *strsep(char **, const char *);
+void *memrchr(const void *, int, size_t);
 #endif
 
 #ifdef __cplusplus

src/string/memrchr.c

+#include <string.h>
+#include "libc.h"
+
+void *__memrchr(const void *m, int c, size_t n)
+{
+	const unsigned char *s = m;
+	c = (unsigned char)c;
+	while (n--) if (s[n]==c) return (void *)(s+n);
+	return 0;
+}
+
+weak_alias(__memrchr, memrchr);

src/string/strrchr.c

 #include <string.h>
 
+void *__memrchr(const void *, int, size_t);
+
 char *strrchr(const char *s, int c)
 {
-	const char *p;
-	c = (char)c;
-	for (p=s+strlen(s); p>=s && *p!=c; p--);
-	return p>=s ? (char *)p : 0;
+	return __memrchr(s, c, strlen(s));
 }