syntax-check: Simplify and fix unmarked_diagnostics

The check was concerning itself with whitespace where it
didn't need to, and used some confusing escaping for one
of its regular expressions - which GNU sed was fine with,
but FreeBSD's sed didn't like one bit.

Switch to extended regular expressions (which, incidentally,
were already in use in the same rule when calling grep) and
remove all whitespace handling.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>