parse-options.[ch]: make opt{bug,name}() "static"

parse-options.[ch]: make opt{bug,name}() "static"

Change these two functions to "static", the last user of "optname()"
outside of parse-options.c itself went away in the preceding commit,
for the reasons noted in 9440b831ad5 (parse-options: replace
opterror() with optname(), 2018-11-10) we shouldn't be adding any more
users of it.

The "optbug()" function was never used outside of parse-options.c, but
was made non-static in 1f275b7c4ca (parse-options: export opterr,
optbug, 2011-08-11). I think the only external user of optname() was
the commit-graph.c caller added in 09e0327f57 (builtin/commit-graph.c:
introduce '--max-new-filters=<n>', 2020-09-18).

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