MFC r256800: pathchk: Ensure bytes >= 128 are considered non-portable

MFC r256800: pathchk: Ensure bytes >= 128 are considered non-portable
characters.

This was not broken on architectures such as ARM where char is unsigned.

Also, remove the first non-portable character from the output. POSIX does
not require this, and printing the first byte may yield an invalid byte
sequence with UTF-8.

PR:		165988
Reported by:	Nicolas Rachinsky
Relnotes:	yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@270890 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f