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/main.c

@@ -922,10 +922,7 @@ int main(int argc, char * argv [])
                       progname,
                       (upd->op == DEVICE_READ)? 'r': (upd->op == DEVICE_WRITE)? 'w': 'v',
                       upd->filename, mtype);
-      if ((upd->memtype = strdup(mtype)) == NULL) {
-        avrdude_message(MSG_INFO, "%s: out of memory\n", progname);
-        exit(1);
-      }
+      upd->memtype = cfg_strdup("main()", mtype);
     }
 
     if (!avr_mem_might_be_known(upd->memtype)) {