When a child fails without exec'ing, we want a well-known status;
best is to match what env(1), nice(1), su(1), and other wrapper
programs do.  This patch adds enum values that later patches will
use, and sets up virFork as the first client of EXIT_CANCELED
for errors detected prior to even attempting exec, as well as
virExec to distinguish between a missing executable vs. a binary
that cannot be executed.

This is a slight semantic change in the unlikely case of a child
process failing to restore its signal mask - we now kill the
child with a known status instead of relying on the caller to
notice and do an appropriate _exit().  A subsequent patch will
make further cleanups based on an audit of all callers.

* src/internal.h (EXIT_CANCELED, EXIT_CANNOT_INVOKE)
(EXIT_ENOENT): New enum.
* src/util/vircommand.c (virFork): Document specific exit value if
child aborts early.
(virExec): Distinguish between various exec failures.
* tests/commandtest.c (test1): Enhance test.
(test22): New test.
Signed-off-by: NEric Blake <eblake@redhat.com>