escape: introduce decescape

This 3-digit decimal escape style is useful when presenting DNS RR
information in zone-file presentation format.

src/basic/escape.h

@@ -65,6 +65,7 @@ static inline char* xescape(const char *s, const char *bad) {
         return xescape_full(s, bad, SIZE_MAX, 0);
 }
 char* octescape(const char *s, size_t len);
+char* decescape(const char *s, const char *bad, size_t len);
 char* escape_non_printable_full(const char *str, size_t console_width, XEscapeFlags flags);
 
 char* shell_escape(const char *s, const char *bad);