Let base64_encode() take a char *

Throughout the whole code base only char * is passed, but a unsigned
char * is expected. This leads to several warnings.

Fix the interface and cast to unsigned char * internally to avoid any
changes to the behaviour.

proto.h

@@ -831,4 +831,4 @@ void srand48(long);
 long lrand48(void);
 #endif
 
-extern Str base64_encode(const unsigned char *src, size_t len);
+extern Str base64_encode(const char *src, size_t len);