GPG user ID matching: use case-insensitive matching

GPG user ID matching: use case-insensitive matching

Although not mentioned in the GPG documentation at [1], GPG uses
case-insensitive matching also for the '<' (exact e-mail) and '@'
(partial e-mail) operators. Matching for '=' (full exact match) is
case-sensitive. Compare [2].

[1] https://www.gnupg.org/documentation/manuals/gnupg/Specify-a-User-ID.html
[2] https://dev.gnupg.org/source/gnupg/browse/master/g10/keyring.c;22f7dddc34446a8c3e9eddf6cb281f16802351d7$890

Bug: 547789
Change-Id: I2f5ab65807d5dde3aa00ff032894701bbd8418c9
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>