stdbuf: incorrect exit status when command can not be invoked

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit 20b5365), GNU coreutils v8.30

Steps to reproduce:

$ stdbuf -o1 .

What happens now: with uutils stdbuf, the exit status is 1.

What I expected to happen: with GNU stdbuf, the exit status is 126.

Notes: this is causing a failure in the GNU test file tests/misc/stdbuf.sh.

For reference, the documentation for GNU stdbuf states

Exit status:

• 125 if ‘stdbuf’ itself fails
• 126 if COMMAND is found but cannot be invoked
• 127 if COMMAND cannot be found
• the exit status of COMMAND otherwise

Finally, for bonus points, the error message is slightly different. uutils says

stdbuf: failed to execute process: Permission denied

and GNU says

stdbuf: failed to run command ‘.’: Permission denied