FETCH_CMD and dependencies

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
currently it is not possible for the pkgsrc infrastructure to depend on
the correct packages for FETCH_CMD. I would like to obsolete the current
FETCH_CMD usage and replace it with FETCH_USING as follows.

FETCH_USING has a value of ftp/fetch/wget/curl/manual. The default is
either ftp or fetch (to be discussed separately). If FETCH_USING is not
manual, it will add a bootstrap dependency on the necessary package.
If FETCH_USING is set to manual, FETCH_CMD specifies to program to use,
FETCH_BEFORE_ARGS etc have to be provided in that case as well.

A check in bsd.prefs.mk will bail out explicitly if FETCH_CMD is set,
but FETCH_USING is not manual. Is that too restrictive?

Do we want to have a fallback mechasnim with associated magic so that
fetching wget when using FETCH_USING=wget does use ftp or fetch?

Joerg