Output strerror from xo_warn

[alfred]

Summary

Right now xo_warn() causes problems because it doesn't output the error number any longer.

This is what xo_warn should output:

/usr/src/lib/libxo % wc /nonexist
wc: /nonexist: open: No such file or directory

However libxo is broken and only outputs this:

/usr/src/lib/libxo % wc /nonexist         
wc: /nonexist: open

[bapt]

Looks good to me

[marcel]

bapt@ already approved, but let me give a thumbs up as well.

[alfred]

Closed by commit rS276273 (authored by alfred).