Add my_str_malloc and _free function pointers to strings library

which will by default exit(1) if malloc() fails, but can be set
to do something else by the calling program does, which mysqld
does to use my_malloc(..., MYF(MY_FAE)) instead. Also checks
allocation in conf_to_src utility program. (Bug #7003)

strings/conf_to_src.c

@@ -75,6 +75,8 @@ static int get_charset_number(const char *charset_name)
 char *mdup(const char *src, uint len)
 {
   char *dst=(char*)malloc(len);
+  if (!dst)
+    exit(1);
   memcpy(dst,src,len);
   return dst;
 }