ao_alsa: move parameter append code to a function

Why not. (I thought I needed this, but my other experiments failed. So
this is merely a minor cleanup.)
player: fix exiting if both audio and video fail initializing

The player was supposed to exit playback if both video and audio failed
to initialize (or if one of the streams was not selected when the other
stream failed). This didn't work; for one this check was missing from
one of the failure paths. And more importantly, both checked the
current_track array incorrectly.

Fix these issues, and move the failure handling code into a common
function.

CC: @mpv-player/stable
terminal: strictly don't read terminal input if stdout is not a terminal

Doing that doesn't make sense anyway: it's meant for interactive input,
and if the output of the player is not on the terminal, how will you
interact with it?

It was also quite in the way when trying to read verbose output with
e.g. less while the player was running, because the player would grab
half of all input meant for less (simply because stdin is still
connected to the terminal).

Remove the now redundant special-casing of pipe input.
terminal: drop ncurses/terminfo/termcap support

It was disabled since the last release, and nobody complained loudly.

Further details see commit 4b5c3ea7.