Tag game-music-emu (libgme) 0.6.3 in preparation for an ABI break.

Tag 0.6.2, containing a security fix for NSFE,

and some minor additional hardening in NSFE.  This is to address
issue #14, assigned CVE-2017-17446.

This is a retag in the source control system since commits between 0.6.1
and the then-current master were accidentally rebased into the old tag
prior to official release.

Tag game-music-emu 0.6.1

Manually tag the old 0.6.0 version.

Manually tag the old 0.5.5 version.