input: ignore modifiers when releasing keys

This prevents keys from getting stuck if the modifier is released before
the base key.
input: check for abort cmd in multi-commands

MP_CMD_COMMAND_LIST commands (used to implement key bindings with
multiple commands) were not checked for abort commands. Implement it.

Remove the remarks about multi-commands being special from the manpage.
Seek coalescing is handled differently now, and the issue with abort
commands is fixed with this commit.