ipc: report some user errors better

Using the IPC with a program, it's not often obvious that a newline must
be sent to terminate a command. Print a warning if the connection is
closed while there is still uninterpreted data in the buffer.

Print the OS reported error if reading/writing the socket fails. Print
an erro if JSON parsing fails.

I considered silencing write errors if the write end is closed (EPIPE),
because a client might send a bunch of commands, and then close the
socket without wanting to read the reply. But then, mpv disconnects
without reading further commands that might still be buffered, so it's
probably a good idea to always print the error.
client API: document requirement to block SIGPIPE

I noticed that the IPC code does not use MSG_NOSIGNAL or SO_NOSIGPIPE.
The former is "only" POSIX 2008 and also requires switching to sendto(),
while the latter is even less portable.

Not going to bother with this obsolete 80ies crap, just block SIGPIPE,
and instruct client API users to do the same.
ipc: add enable_event and disable_event commands

This was requested.
stream: always disable cache for pseudo-streams

Streams which don't have a full_buffer function never return any actual
data. Slight improvement over commit 5640c195.
player: fix a typo in message output

This typo has been around for over a year. Oops.
player: hack against --keep-open misbehaving with broken files

If a file (or a demuxer) is broken, seeking close to the end of the file
doesn't work, and seek_to_last_frame() will be called over and over
again, burning CPU for no reason.

Observed with incomplete mp4 files. That this can happen was already
mentioned in commit 090f6cfc, but I guess now I'll do something against
it.

hrseek_lastframe is cleared by reset_playback_state(), so it's only set
if seek_to_last_frame() was called, and no other seek happened since
then. If finding the last frame succeeds, no EOF will happen (unless the
user unpauses, but then it will simply remain at the last frame). If it
fails, then it will return immediately, without retrying.