some improvement for systemctl3.py

[ladyrick]

1. Fix SyntaxWarning: invalid escape sequence '\w'

2. Support multiple cmd path. Sometimes the "cat" command is not "/usr/bin/cat". So I change CAT_CMD to a list of candidates CAT_CMDS and search for the first existing path. Same change for LESS_CMD and TAIL_CMD.

3. In log_unit_from, instead of spawnvp a new process for cat/tail/less, I use execvp to replace current process. By doing this, systemctl needn't to handle the exit code and errors.
   For example, before this PR, when I use systemctl log -f service to see the log and then use ctrl+c to exit, I will get an ugly error like this. But when this PR is accepted, the tail command will exit directly and no error will show.

[gdraheim]

Please add get_exist_path to systemctl3.pyi

[ladyrick]

Please add get_exist_path to systemctl3.pyi

Done.

[gdraheim]

Ahh, mypy asks for explicit return None in get_exist_path(paths):

[ladyrick]

Ahh, mypy asks for explicit return None in get_exist_path(paths):

Nice, All checks have passed this time.

[gdraheim]

I will do some more checks locally before pushing to master though. In any case, thanks a lot for the patch, that is really helpful for the project to continue.