util: Add virProcessGetStat

util: Add virProcessGetStat

This reads and separates all fields from /proc/<pid>/stat or
/proc/<pid>/task/<tid>/stat as there are easy mistakes to be done in the
implementation.  Some tests are added to show it works correctly.  No number
parsing is done as it would be unused for most of the fields most, if not all,
of the time.  No struct is used for the result as the length can vary (new
fields can be added in the future).

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>