lib: add vty_frame() to get rid of unneeded config

lib: add vty_frame() to get rid of unneeded config

vty_frame() can be used to reduce the amount of output produced by "show
running-config" and "write ...".  It buffers output in struct vty->frame
(1024 bytes) and outputs it when vty_out is called.  If vty_out isn't
called, it can be removed with vty_endframe() later.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>