Remove `Word` from the `unix_sigpipe` attribute template so that plain
`#[unix_sigpipe]` is not included in suggestions of valid forms of the
attribute. Also re-arrange diagnostics code slightly to avoid duplicate
diagnostics.
The test is for the "bare" variant of the attribute that looks like this:
#[unix_sigpipe]
which is not allowed, because it must look like this:
#[unix_sigpipe = "sig_ign"]