Support the ".if v" conditional operator (vroff mode, always false)

for groff compatibility because pod2man(1) uses it that way.
Weirdly, groff documents it as "for compatibility with other
troff versions" but neither Heirloom nor Plan 9 have it.
Issue reported by giovanni@ via sthen@.

1 files changed, 3 insertions, 1 deletions

diff --git a/roff.7 b/roff.7
index 30568620..6998a133 100644
--- a/roff.7
+++ b/roff.7
@@ -747,9 +747,11 @@ If the first character of COND is
 .Pq even page ,
 .Sq r
 .Pq register accessed ,
-or
 .Sq t
 .Pq troff mode ,
+or
+.Sq v
+.Pq vroff mode ,
 COND evaluates to false.
 .It
 If COND starts with a parenthesis or with an optionally signed