Fix default GPGHOMEDIR setting in libgpgme context.
Fix default GPGHOMEDIR setting in libgpgme context.

GPGME was using the process owner's default GPGHOMEDIR directory,
regardless of any keyfiles specified, meaning that if there were any
other non-signing enabled secret keys in the keyrings, and one of the
other keys was first on the secret keyring, that first key would be used
for signing rather than the secret key in the specified keyfile.

 * FIX problem detailed in comment 11 on #5463:
   https://trac.torproject.org/projects/tor/ticket/5463#comment:11

mmaker/bridgedb,mmaker/bridgedb,pagea/bridgedb,pagea/bridgedb