kill: kill old form with signal name
kill: kill old form with signal name

This commit aim to correct #2645.
The origin of the problem was that `handle_obsolete` was not looking for
named signals but only for numbers preceded by '-'. I have made a few
changes:

- Change `handle_obsolete` to use `signal_by_name_or_value` to parse the
  possible signal.
- Since now the signal is already parsed return an `Option<usize>`
  instead of `Option<&str>` to parse later.
- Change the way to decide the pid to use from a `match` to a `if`
  because the tested element are actually not the same for each branch.
- Parse the signal from the `-s` argument outside of `kill` function for
  consistency with the "obsolete" signal case.
- Again for consistency, parse the pids outside the `kill` function.

uutils/coreutils,uutils/coreutils,uutils/coreutils