setup.ini signing key rotation

setup.ini signing key rotation

Verify the setup.ini signature using a new key, or the old key (unless
the --disable-old-keys option is used)

New key:

sec   4096R/1A698DE9E2E56300 2020-02-27 [expires: 2022-02-26]
      Key fingerprint = 5640 5CF6 FCC8 1574 682A  5D56 1A69 8DE9 E2E5 6300
uid                          Cygwin <cygwin@cygwin.com>

Old key:

sec   1024D/A9A262FF676041BA 2008-06-13
      Key fingerprint = 1169 DF9F 2273 4F74 3AA5  9232 A9A2 62FF 6760 41BA
uid                          Cygwin <cygwin@cygwin.com>
ssb   1024g/0AF098B5A1DB7B5C 2008-06-13

Export key from gpg and generate s-expr format using:

$ gpg --export KEYID >cygwin.pub
$ ./gpg-key-to-s-expr.sh -C ./cygwin.pub >cyg-pubkey.h