Implement dev option -c */[ASsrt] skeleton

Also changed usbdev, usbsn, usbvendor and usbproduct components from
PROGRAMMER structure to be cached string pointers rather than fixed-size
arrays. These will be initialised by pgm_new() with a pointer to nul;

src/config.h

@@ -101,8 +101,6 @@ void pyytext(void);
 
 char * dup_string(const char * str);
 
-char * cache_string(const char * file);
-
 #ifdef __cplusplus
 }
 #endif