Improve return value of OS.execute in blocking/non-blocking variants

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.

0 files changed, 0 insertions, 0 deletions