Change: Try stopping extmidi player with SIGINT first. (#11404)

This may give the player a chance to issue MIDI note-off commands.

The kill/waitpid cycle is also less aggressive, waiting 50ms each time.

Fix: extmidi did not move on to next song after playing ends. (#11469)

`song` is no longer a C-style string so cannot be cleared by writing a NUL char.
Use `.clear()` to properly clear a std::string.

Codechange: Remove duplicated includes. (#10888)

These are now included by stdafx.h so don't need to be included again.

Codechange: Pass a MusicSongInfo struct instead of bare filename to music drivers.

Preparation for later extending the info passed to music drivers.