Harden string processing during parsing in lexer.l, config_gram.y and otherwise

- Replace strdup(s) with cfg_strdup(funname, s) that exits on out of mem
 - Replace malloc(n) with cfg_malloc(funname, n) that exits on out of mem
 - Change multiline string scanning in lexer.l to avoid core dump
 - Remove global variables string_buf and string_bug_ptr
 - Ensure reading strings unescapes strings C-Style
 - Ensure writing strings escapes strings C-Style again

Commit looks longer than needed as unescape() and auxiliary functions needed
to be moved from term.c (not in libavrdude) to config.c (in libavrdude).

src/avr.c

@@ -1243,7 +1243,7 @@ void avr_add_mem_order(const char *str) {
     if(avr_mem_order[i] && !strcmp(avr_mem_order[i], str))
       return;
     if(!avr_mem_order[i]) {
-      avr_mem_order[i] = strdup(str);
+      avr_mem_order[i] = cfg_strdup("avr_mem_order()", str);
       return;
     }
   }