Fix file_name_is_absolute on VMS for device without a directory.

To be considered absolute, we had been requiring a file spec to
have a bracketed directory spec after the colon. This meant that
very common and idiomatic expressions such as sys$login:login.com
or sys$manager:operator.log were not considered absolute.  Which
is wrong.

So we now consider a file spec starting with a valid device name
(which would also be a valid logical name) followed by an unescaped
colon to be absolute.