Include {private,public}_key.inc in gpg test SOURCES.
Get rid of ifdefs in gpg tests.
Include config.site/coverage into dist.
Store test keys as plain files.

And the key now does not have a passphrase set.
Basic infrastructure for gpg tests.
authinfo_read_file test.
Test for files with encrypted passwords.
Test for badly base64-encoded passwords.
Remove unused base64_encode function.