env: signal flags do not understand `RTMIN+n`

[enr0n]

This probably affects all tools that use signals in some way. First, signal.rs does not consider SIGRTMIN or SIGRTMAX on Linux at all.

But, even where these are defined, the code does not handle SIGRTMIN+n notation. E.g.:

root@r:~# env --block-signal=RTMIN+7
env: 'RTMIN+7': invalid signal
root@r:~# env --version
uu_env (uutils coreutils) 0.6.0

[collinfunk]

None of the programs in uutils seem to handle realtime signals. See a further example of --ignore-signal misbehaving:

$ ./target/debug/env --ignore-signal yes > /dev/null &
[1] 74104
$ ./target/debug/kill -RTMIN 74104
error: unexpected argument '-R' found

  tip: to pass '-R' as a value, use '-- -R'

Usage: kill [OPTIONS]... PID...

For more information, try '--help'.
$ env kill -RTMIN 74104
[1]+  Real-time signal 0         ./target/debug/env --ignore-signal yes > /dev/null

GNU kill shouldn't be able to use a realtime signal to kill yes in that case.

$ env --ignore-signal yes > /dev/null &
[1] 74197
$ env kill -RTMIN 74197
$ ps 74197
    PID TTY      STAT   TIME COMMAND
  74197 pts/0    R      0:45 yes
$ kill -KILL 74197
[1]+  Killed                     env --ignore-signal yes > /dev/null