bash-completion: update chrt completion

bash-completion: update chrt completion

Add couple missing options, and make the completion overall work better.

That said completion is still incomplete, pardon the pun.  After user has
specified policy then giving a hint what priority needs to be specified is
theoretically possible, but such hint is not given.  There does not seem to
be easy way to know when user wants stops specifying options and move to
defining priority in: chrt [options] [prio] [command|pid].

Signed-off-by: Sami Kerola <kerolasa@iki.fi>