reflog_expire(): new function in the reference API

reflog_expire(): new function in the reference API

Move expire_reflog() into refs.c and rename it to reflog_expire().
Turn the three policy functions into function pointers that are passed
into reflog_expire(). Add function prototypes and documentation to
refs.h.

[jc: squashed in $gmane/261582, drop "extern" in function definition]

Signed-off-by: Michael Haggerty <redacted>
Reviewed-by: Stefan Beller <redacted>
Tweaked-by: Ramsay Jones
Signed-off-by: Junio C Hamano <redacted>