gpg: Add --list-filter properties key_expires and key_expires_d.

* g10/import.c (impex_filter_getval): Support new filter properties.

Here is how to list all subkeys expiring in the year 2061:

gpg --list-keys --list-filter

'select= sub/key_expires_d -gt 2061-01-01 \
         && sub/key_expires_d -lt 2061-12-31'

To list all primary key expirations, use the "pub/" prefix and to list
all expiration dates use no prefix.

• GnuPG-bug-id: T6509