options: change handling of "no-" options

Normally, all flag options can be negated by prepending a "no-", for
example "--no-opt" becomes "--opt=no". Some flag options can't actually
be negated, so add a CONF_TYPE_STORE option type to disallow the "no-"
fallback.

Do the same for choice options. Remove the explicit "no-" prefixed
options, add "no" as choice.

Move the handling of automatic "no-" options from parser-mpcmd.c to
m_config.c, and use it in m_config_set_option/m_config_parse_option.
This makes these options available in the config file. It also
simplifies sub-option parsing, because it doesn't need to handle "no-"
anymore.
config: do not require option value

Now setting a value with "=" is not required anymore in config files.
This should work analogous to command line arguments. Putting an entry
"opt=value" into the config file is like "--opt=value" on the command
line, and "opt" is like "--opt=" and "--opt".
options: unify single dash and double dash options

There were two option syntax variations:

    "old":  -opt value
    "new": --opt=value

"-opt=value" was invalid, and "--opt value" meant "--opt=" followed by
a separate option "value" (i.e. interpreted as filename). There isn't
really any reason to do this. The "old" syntax used to be ambiguous
(you had to call the option parser to know whether the following
argument is an option value or a new option), but that has been removed.
Further, using "=" in the option string is always unambiguous.

Since the distinction between the two option variants is confusing,
just remove the difference and allow "--opt value" and "-opt=value".

To make this easier, do some other cleanups as well (e.g. avoid having
to do a manual lookup of the option just to check for M_OPT_PRE_PARSE,
which somehow ended up with finally getting rid of the m_config.mode
member).

Error reporting is still a mess, and we opt for reporting too many
rather than too few errors to the user.

There shouldn't be many user-visible changes. The --framedrop and
--term-osd options now always require parameters.

The --mute option is intentionally made ambiguous: it works like a flag
option, but a value can be passed to it explicitly ("--mute=auto"). If
the interpretation of the option is ambiguous (like "--mute auto"), the
second string is interpreted as separate option or filename. (Normal
flag options are actually ambiguous in this way too.)
cleanup: replace OPT_FLAG_ON and OPT_MAKE_FLAGS with OPT_FLAG

OPT_MAKE_FLAGS() used to emit two options (one with "no" prefixed),
but that has been long removed by special casing flag options in the
option parser. OPT_FLAG_ON() used to imply that there's no "no-"
prefixed option, but this hasn't been the case for a while either.
(Conceptually, it has been replaced by OPT_FLAG_STORE().)

Remove OPT_FLAG_OFF, which was unused.
mplayer: remove seconds/centiseconds display from terminal status line

Doesn't have much of a purpose for normal playback. You can get
milliseconds display with --osd-fractions. It's also possible to build
a custom status line with --status-msg.

This gives more space on the status line and, in my opinion, is a bit
less annoying.
Check return values of some mp_find_..._config_file function calls for NULL
options: change --no-config option, make it apply to input.conf as well

Simplify --no-config and make it a normal flag option, and doesn't take
an argument anymore. You can get the same behavior by using --no-config
and then --include to explicitly load a certain config file.

Make --no-config work for input.conf as well. Make it so that
--input:conf=file still works in this case. As a  technically unrelated
change, the file argument now works as one would expect, instead of
making it relatively to "~/.mpv/". This makes for simpler code and
easier to understand option semantics. We can also print better error
messages.