alphasort app names in keysync --help output
add gpg as supported app, KeySync reads DSA subkeys into the keydict

This is very preliminary support, only reading and only private keys.

closes #1872
update man page to reflect new I/O flow and new options (fixes #2106)
small step towards keysync and keysync-gui having the same convert process

Ultimately, keysync and keysync-gui should be calling the same function to
run the conversion so we know that they are doing the same thing.
gnupg: rename files from 'gpg' to 'gnupg' since that's the canonical name

The software package is called GnuPG, as in http://gnupg.org. gpg is but
one command in the GnuPG suite.  I suppose we could go with Gnu Privacy
Guard, but I think few people call it that.
gnupg: rename GPGProperties to GnuPGProperties
improve gvfs detection to allow systems with gvfs to also use pymtp

* the logic was broken, using a test of 'darwin' to decide when to use MTP
* this now tests whether the GVFS MTP is actually active, and if not it
  then uses the pymtp method of transferring. This means it'll still sync
  on gvfs systems when the gvfs MTP mount has been ejected, but the device
  is still plugged in.
* this also broke on systems without gvfs but with pymtp, fixes #2120
rename get_mtp_mount_path to get_keystore_savedir for clarity