deprecate --fetch-cmd option of bootstrap script

["OBATA Akio" <obache%netbsd.org@localhost>]

Hi,

Now, bootstrap script accept --fetch-cmd option, and set its argument as 
FETCH_CMD.
But it is not works nowadays, because

1. fetch command will be selected as FETCH_USING
2. FETCH_CMD will only be used with FETCH_USING=custom
3. appropriate FETCH_BEFORE_ARGS, FETCH_AFTER_ARGS, FETCH_RESUME_ARGS and 
FETCH_OUTPUT_ARGS
   also must be defined for FETCH_USING=custom

So I feel this option should be gone away.
It may be nice introducing --fetch-using option for set default FETCH_USING 
variable.

Any comments and suggestions?

--
OBATA Akio / obache%NetBSD.org@localhost