input: use embedded etc/input.conf copy for default bindings
Remove the def_cmd_binds[] table which stored default key bindings
from input.c. Instead, embed a copy of etc/input.conf in the binary
and parse that at runtime. Also simplify some of the section-handling
code and remove unused code.
This is mostly an internal change to simplify maintenance, but there
are a couple of minor user-visible differences. The current contents
of etc/input.conf should match what input.c defined before, except
that it defines bindings unconditionally while input.c had some
bindings using conditional functionality under #ifdef; the only effect
of that difference should be changing a warning about an unbound key to
one about an unrecognized command. For user input.conf entries using
the section functionality, whitespace inside braces is now ignored
(thus "{ dvdnav }" now means the same as "{dvdnav}"). User-defined
keys now work in --idle mode before any file has been played; before
only internal default bindings worked until mp_input_set_section() was
called once, which only happened at the start of the first file.