run command: avoid using 1 as special value

run command: avoid using 1 as special value

In Perl, the last expression of a block (e.g. of a method, eval) gets
returned if there's no explicit return statement. Quite often that is
truthy, i.e., 1.

As that was chosen as the special value for the CMD_FINISHED flag it
had quite a few false positives, causing weird effects and
installation failure.

Reserve that overly problematic value and chose 2 as new CMD_FINISHED
value, albeit it could be better to signal this even more explicitly,
like with a structured hash reference, but for now this is a good stop
gap.

Fixes: 23c5fbe ("sys: command: allow terminating the process early from log subroutine")
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>