Add unescape() to libutil

formerly known as resolveescapes(), it is of central use to numerous
programs.
This drops a lot of LOC.

util.h

@@ -56,3 +56,4 @@ char *humansize(double);
 mode_t parsemode(const char *, mode_t, mode_t);
 void putword(const char *);
 void recurse(const char *, void (*)(const char *));
+size_t unescape(char *);