*: make DEFUN installations file-local

*: make DEFUN installations file-local

This moves all install_element calls into the file where the DEFUNs are
located.  This fixes several small related bugs:

- ospf6d wasn't installing a "no interface FOO" command
- zebra had a useless copy of "interface FOO"
- pimd's copy of "interface FOO" was not setting qobj_index, which means
  "description LINE" commands would fail with an error

The next commit will do the actual act of making "foo_cmd" static.

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