man: fix pic issue at the source

The commit 0993b66 (man: pic failed to run during manpage-check) worked
around the manpage-check warning generated by groff.  Using "-T ascii"
rather "-T utf8" was enough to silence the warning because the man page
has this condition in it:

  .if !'\*[.T]'ascii'

However, rpmlint generates the same warning as manpage-check was (it
uses -Tutf8), and manpages are generated using -Tutf8 (leading to an
fairly unreadable drawing).  So let's change the logic a bit and allow
pdf generation w/ nice drawing and kill it for tty's.

Cc: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>