fix revoke_key typo in creating temporary directory

fix revoke_key typo in creating temporary directory
fix variable specifying which key to revoke

monkeysphere-host revoke-key <key-id> would produce the following errors, this
commit fixes that:

Really publish this cert to zimmermann.mayfirst.org ? (Y/n) y
/usr/share/monkeysphere/mh/revoke_key: line 96: mkmstempdir: command not found
gpg: new configuration file `/root/.gnupg/gpg.conf' created
gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run
gpg: "0x!" not a key ID: skipping