memcpy_pic: reformat

Besides cosmetic changes, also change memcpy_pic return type and remove
config.h include.
input.conf: clarify the magic how default key bindings are added

This is a confusing mechanism, so the explanation should bit more clear.
osdep: handle SIGTERM

There's no reason why we should e.g. handle SIGQUIT, but not SIGTERM.

Note that sending SIGTERM twice still kills the player.
ao_oss: make no_persistent_volume volume work when seeking
ao_oss: don't enable -softvol by default on OSSv4
ao_oss: hide warning