KEYS: Provide PGP key description autogeneration

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

-------------------------------------------------

Provide a facility to autogenerate the name of PGP keys from the contents
of the payload.  If add_key() is given a blank description, a description
is constructed from the last user ID packet in the payload data plus the
last 8 hex digits of the key ID.  For instance:

	keyctl padd asymmetric "" @s </tmp/key.pub

might create a key with a constructed description that can be seen in
/proc/keys:

2f674b96 I--Q---     1 perm 39390000     0     0 crypto    \
			Sample kernel key 31f0ae93: PGP.RSA 31f0ae93 []

Changelog

v0:
- fix style issues (Roberto Sassu)

Signed-off-by: David Howells <dhowells@redhat.com>
Co-developed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>