chrt: Fix confusing error messages when priority argument is required

chrt: Fix confusing error messages when priority argument is required

While making the priority argument optional for non-prio policies, some
confusing error messages were accidentally exposed to users for cases
that do need a priority argument.

  $ chrt true		#1
  chrt: invalid priority argument: 'true'
  $ chrt 1		#2
  chrt: failed to set pid 0's policy: Operation not permitted
  $ sudo chrt 1		#3
  chrt: no command specified
  $ chrt --other 1	#4
  chrt: unsupported priority value for the policy: 1: see --max for valid range

The error message #1 is caused by mixing `have_prio' and `need_prio'
together. Therefore, it always tries to parse the first positional
argument as a priority when `--pid' is not given. #2 shows that
set_sched() is meaninglessly called even when too few arguments are
specified.

At first glance, the error message #3 seems to be correct, but it turns
out to be very wrong -- the only positional argument in this case must
be regarded as a command, and commit 223a502b0208 ("chrt: (man,usage)
mark the priority value as optional in the synopses") also clearly
stated the same in the help message and the manual. In other words, #4
should have tried to execute `1' from PATH.

Fix #1 by decoupling `need_prio' from the priority parsing routine.

Fix #2 by parsing the first argument as a priority only when it's not
the only argument.

Fix #3 and #4 by consuming optind immediately when parsing priority
argument, instead of postponing it with inconsistent conditions (I have
an intuition that the previous code path was vibe-coded...)

Now #1 returns

  chrt: policy SCHED_RR requires a priority argument

... #2, #3 return

  chrt: no command or priority specified

... and #4 returns

  chrt: failed to execute 1: No such file or directory

... which are more sensible and helpful.

This doesn't break existing usage patterns:

  $ chrt --other true
  $ chrt --other 0 true
  $ chrt --other 1 true
  chrt: unsupported priority value for the policy: 1: see --max for valid range
  $ chrt --other echo meow
  meow

Fixes: 4c425142844d ("chrt: Allow optional priority for non‑prio policies without --pid")
Signed-off-by: Rong Zhang <i@rong.moe>