Protect gnupg invocations with a mutex
Protect gnupg invocations with a mutex

gnupg does not work reliably when multiple processes are using the
keyring at once (at least on OS X).

Example error (from Travis) without the lock:

    warning: Feed http://0install.net/2006/interfaces/0publish: Failed to check feed signature:
    Error checking signature for 'http://0install.net/2006/interfaces/0publish': GPG failed:
    gpg: starting migration from earlier GnuPG versions
    gpg: porting secret keys from '/Users/travis/.gnupg/secring.gpg' to gpg-agent
    gpg: migration succeeded
    gpg: can't open '/Users/travis/.gnupg/pubring.gpg'
    gpg: keydb_get_keyblock failed: Value not found

bastianeicher/0install,bastianeicher/0install,bastianeicher/0install,0install/0install,0install/0install,0install/0install