The unsigned long option parsing (including 'k'/'m'/'g' suffix
parsing) is more widely applicable.  Add support for OPT_MAGNITUDE
to parse-options.h and change pack-objects.c use this support.

The error behavior on parse errors follows that of OPT_INTEGER.  The
name of the option that failed to parse is reported with a brief
message describing the expect format for the option argument and
then the full usage message for the command invoked.

This differs from the previous behavior for OPT_ULONG used in
pack-objects for --max-pack-size and --window-memory which used to
display the value supplied in the error message and did not display
the full usage message.
Signed-off-by: NCharles Bailey <cbailey32@bloomberg.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>