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)
- GnuPG-bug-id: T1715