Replace string arrays with const char * and allocated space (part 2)

This commit replaces fixed-string buffers in PROGRAMMER, AVRPART and AVRMEM
that are dealt with by the parser and grammar. Now, string assignments are
always to const char *, ie, these are read-only strings with arbitrary
length.

config_gram.y now only needs to consider one type of string assignment.

This commit also

  - Replaces the simple linear-search cache_string() function with faster
    hashed cache_string(). Either way, the returned value is likely to be
    shared, so should never be free()'d.

  - Duplicates hvupdi_support list in pgm_dup() and frees it in pgm_free()

  - Adds const qualifier to some function args in avrpart.c and pgm.c

  - Hardens some functions against being called with NULL pointers

  - Ensures _new() and _dup() functions for parts, programmers and memory
    return a suitable memory. Out of memory triggers exit in one of three
    functions, cfg_malloc(), cfg_realloc() and cfg_strdup(); there is
    rarely anything useful that AVRDUDE or, for that matter, any
    application compiled against libavrdude can do once you run out of
    memory as AVRDUDE/libavrdude rely heavily on allocation of memory.

src/config.h

@@ -94,7 +94,7 @@ void print_token(TOKEN *tkn);
 
 void pyytext(void);
 
-int capture_comment_char(int c);
+void capture_comment_str(const char *str);
 
 #ifdef __cplusplus
 }