[Groff] new r/o register `.ns'

[Werner LEMBERG]

I've implemented a new read-only number register `.ns' which returns 1
in no-space mode (if in top-level diversion) and 0 otherwise.


    Werner


PS: Today morning I've checked in this change with `.q' as the
    register name.  Right now I've changed this to `.ns' to avoid
    possible conflicts with two-character register names of Unix
    troff.