Initialized the PID to -2, which will be the value returns in blocking-
mode where the PID is not available. (-1 was already taken to signify an
execution failure).
OS::execute will now properly return a non-OK error code when it fails
to execute the target file.
The documentation was rewritten to be very clear about the differences
between blocking and non-blocking mode.
Fixes#19056.
(cherry picked from commit f392650be2)
