fsck: make fsck_config() re-usable

fsck: make fsck_config() re-usable

Move the fsck_config() function from builtin/fsck.c to fsck.[ch]. This
allows for re-using it in other tools that expose fsck logic and want
to support its configuration variables.

A logical continuation of this change would be to use a common
function for all of {fetch,receive}.fsck.* and fsck.*. See
5d477a334a6 (fsck (receive-pack): allow demoting errors to warnings,
2015-06-22) and my own 1362df0d413 (fetch: implement fetch.fsck.*,
2018-07-27) for the relevant code.

However, those routines want to not parse the fsck.skipList into OIDs,
but rather pass them along with the --strict option to another
process. It would be possible to refactor that whole thing so we
support e.g. a "fetch." prefix, then just keep track of the skiplist
as a filename instead of parsing it, and learn to spew that all out
from our internal structures into something we can append to the
--strict option.

But instead I'm planning to re-use this in "mktag", which'll just
re-use these "fsck.*" variables as-is.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>