gpg: Print a warning if the subkey expiration may not be what you want.

* g10/keyedit.c (subkey_expire_warning): New.
keyedit_menu): Call it when needed.

The heuristic to detect a problem is not very advanced but it should
catch the most common cases.

(backported from commit ae3d1bbb65b65cf3c57bb14886be120f5e31635d)

[dkg: rebased to STABLE-BRANCH-1-4]

• GnuPG-bug-id: T1715
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>