vtysh: detangle configuration writes

vtysh: detangle configuration writes

vtysh has a very convoluted and confusing setup where it isn't even
clear which files are written where (since some filenames come
indirectly from loading config).  Detangle.

This also removes writing vtysh.conf.  The file is intended to be
manually edited since it has some vague security concerns (if PAM is
used).

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