bgpd: don't sidestep vty code

bgpd: don't sidestep vty code

afi_header_vty_out is sidestepping the vty code, writing straight to the
output (either stdout or the obuf), which results in newline translation
not being performed.

Easiest fix is replacing it with a macro.  Longer-term, I have some old
code to add "prefaces" to the vty output, planning to dig that up.

Fixes: #949 ("bgpd show running doesn't show new lines")
Reported-by: Lou Berger <lberger@labn.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>