PGPLIB: PGP definitions (RFC 4880)

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

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Provide some useful PGP definitions from RFC 4880.  These describe details
of public key crypto as used by crypto keys for things like signature
verification.

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>