Regular Expression Support

[NIIBE Yutaka]

Hello,

OpenPGP has support of regular expression with trust Signature packets.

GnuPG 2.x only supports the feature when it can find regcomp/regexec
routines in system library.  On Windows, it is not supported.

For this issue, I created a task: https://dev.gnupg.org/T4843
And the branch:
    https://dev.gnupg.org/source/gnupg/history/gniibe%252Fregexp/

My purpose here is to minimize difference between different systems.

I'd like to listen opinions from those who actually have use cases.
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