rtla/cli: Unify and improve handling of invalid option arguments

The current handling of invalid command line option arguments is
inconsistent:

- opt_llong_callback() treats non-numerical input the same as "-1",
  which might or might not be rejected later.
- opt_int_callback() returns -1 on non-numerical input without an error
  message, which makes parsing fail silently (libsubcmd will
  automatically print the usage of the option only, no error message).
- custom callbacks abort command line parsing using fatal(), which
  displays an error message and exits, without libsubcmd printing the
  usage.

Unify this such that all invalid options, regardless of the format,
print an error message similar to the out of range case:

Error: --opt: 'value' is not a valid XY

followed by the usage of the option, e.g.:

$ rtla timerlat hist --period=1us
 Error: --period: '1us' is not a valid number

 Usage: rtla timerlat hist [<options>] [-h|--help]

    -p, --period <us>     timerlat period in us

As this is a libsubcmd help path, all option parsing failures now return
the exit code of 129 (help).

The unified handling is implemented using a new error message helper,
opt_err(), which is called from two new CLI-specific parsing functions,
strtoll_safe() and strtoi_safe(), as well as from custom helpers.

Option callback tests are updated to cover the new behavior.

Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260716144901.1187474-1-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>

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