audio/filter: don't wipe full filter chain if adding a filter fails

There's no need for that, and in fact makes it more likely that it
recovers normally.
audio: refactor some aspects of filter chain setup

There's no real reason why audio_init_filter() should exist. Just use
af_init or af_reinit directly. (We lose a useless message; the same
information is printed in a quite close place with more details.)

Requires less code, and the way the filter chain is marked as having
failed to initialize allows just switching off audio instead of
crashing if trying to insert a volume filter in mixer.c fails, and
recreating the old filter chain fails too.
command: move setting playback speed to a separate function
audio/filter: allow removing filters by label

Although the "af" command already could do this, it seems it's better
to introduce a lower level mechanism for now. This avoids some messy
issues, since that code would recursive call reinit_audio_chain().

To be used by the next commit.
audio: enable pitch correction by default when playing fast

Apparently this is what users want. When playing with normal speed,
nothing is done. When playing slower than normal, resampling is used
instead, because scaletempo (which does the pitch correction) adds
too many artifacts.
player: don't print audio/video init failure message twice

The messages "Audio: no audio" and "Video: no video" could be printed
twice each if initializing them failed. Prevent his silliness.

CC: @mpv-player/stable
manpage: redocument audio delay key bindings

Fixes #1131.
CC: @mpv-player/stable