Per POSIX, execvp() is not safe to call in a child process forked from a
multi-threaded process. We can now replace the execvp() call in the child
process with a call to our fork-safe (async-signal-safe) variant.

Prepare our internal execvpe context on the parent's construction path,
use the context in the child, and release the context in the parent on the
way out, regardless of whether the handler as a whole succeeded or not.
(The context is only a temporary resource.)

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20230323121016.1442655-16-lersek@redhat.com>