execve(2) & fexecve(2) don't have any return value/status
on succesful invocation. au_errno_to_bsm() then places the
default BSM_ERRNO_UNKNOWN's local errno mapping for
"ar->ar_errno". If the errno is not found in bsm_errnos[] in
bsm_errno.c, then we set it as 0 for successful execution.

Also, a minor grammatical correction in bsm_arg.c

Set the audit mask corresponding to ex audit class and try to
audit a successful execution of execve(2), where the forked
process successfully overlays the calling process, then in the
audit record's return token, you'll get something like this:

return,failure: Unknown error: 201,4294967295