gpg: Add new usage option for drop-subkey filters.

* g10/import.c (impex_filter_getval): Add new "usage" property for
drop-subkey filter.

For example, this permits extraction of only encryption-capable
subkeys like so:

gpg --export-filter 'drop-subkey=usage !~ e' --export $FPR

• GnuPG-Bug-id: T4019
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>