builtin/config: convert flags to a local variable

builtin/config: convert flags to a local variable

Both the `do_all` and `use_key_regexp` bits essentially act like flags
to `get_value()`. Let's convert them to actual flags so that we can get
rid of the last two remaining global variables that track options.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>