Add Elgamal decryption to netpgp.  Inspired by (BSD-licensed) the
Elgamal decryption code from Postgresql by Marko Kreen.

% cp config.h f
% netpgp -e f
netpgp: default key set to "d4a643c5"
% netpgp -d < f.gpg > f.netpgp
netpgp: default key set to "d4a643c5"
signature  1024/DSA 8222c3ecd4a643c5 2010-05-19 [EXPIRES 2013-05-18]
Key fingerprint: 3e4a 5df4 033b 2333 219b 1afd 8222 c3ec d4a6 43c5
uid              Alistair Crooks (DSA TEST KEY - DO NOT USE) <agc@netbsd.org>
encryption 2048/Elgamal (Encrypt-Only) a97a7db6d727bc1e 2010-05-19 [EXPIRES 2013-05-18]
netpgp passphrase:
% ls -al f*
-rw-r--r--  1 agc  agc  5730 Nov  6 23:53 f
-rw-------  1 agc  agc  1727 Nov  6 23:53 f.gpg
-rw-r--r--  1 agc  agc  5730 Nov  6 23:54 f.netpgp
% diff f f.netpgp
%

This makes DSA keys into first class citizens, since encryption and
decryption using DSA/Elgamal is now supported.

(agc)