sys: command: factor out kill() + waitpid() from run_command()

sys: command: factor out kill() + waitpid() from run_command()

This moves the kill() + waitpid() combo into a separate subroutine,
avoiding open-coding that sequence. wait_for_process() also handles
properly unkillable process (e.g. in D-state) and avoids completely
locking up the installer in such cases. See [0].

For the latter case, a timeout exists (with a default of 5 seconds) in
which to wait for the process to exit after sending an optional
TERM/KILL signal.

Also while at it, add a few basic tests for run_command().

[0] https://lists.proxmox.com/pipermail/pve-devel/2024-February/061697.html

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>