src: Treat PID as signed

src: Treat PID as signed

This initially started as a fix of some debug printing in
virCgroupDetect. However it turned out that other places suffer
from the similar problem. While dealing with pids, esp. in cases
where we cannot use pid_t for ABI stability reasons, we often
chose an unsigned integer type. This makes no sense as pid_t is
signed.
Also, new syntax-check rule is introduced so we won't repeat this
mistake.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>