ps(1): Add a standard exit status section.

[bcr]

Approved, thanks for working on it.

[0mp]

@mat, @krion, may I have your approval?

[jilles]

This is not wrong, but perhaps some details about different types of errors may be helpful, such as that nonexistent usernames in -U are immediately fatal while an empty result (such as from only nonexistent process IDs in -p or only existing users that do not currently run any process in -U) still writes the header line (if not disabled) and also returns a non-zero exit status.

[0mp]

@jilles do you think we could open another PR for your suggestions?

In D17146#366362, @jilles wrote:

This is not wrong, but perhaps some details about different types of errors may be helpful, such as that nonexistent usernames in -U are immediately fatal while an empty result (such as from only nonexistent process IDs in -p or only existing users that do not currently run any process in -U) still writes the header line (if not disabled) and also returns a non-zero exit status.

I've created a PR for this: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232852