reflog: rename `cmd_reflog_expire_cb` to `reflog_expire_options`

reflog: rename `cmd_reflog_expire_cb` to `reflog_expire_options`

We're about to expose `struct cmd_reflog_expire_cb` via "reflog.h" so
that we can also use this structure in "builtin/gc.c". Once we make it
accessible to a wider scope though it becomes awkwardly named, as it
isn't only useful in the context of a callback. Instead, the function is
containing all kinds of options relevant to whether or not a reflog
entry should be expired.

Rename the structure to `reflog_expire_options` to prepare for this.

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