misc/emacs-libvterm: Install etc/ under the vterm/

Previously, the shell-integration scripts (emacs-vterm-bash.sh,
emacs-vterm-zsh.sh, emacs-vterm.fish) were installed under
${EMACS_SITE_LISPDIR}/etc/, i.e. share/emacs/site-lisp/etc/. That is a
shared, generically-named directory, and creating it cause problems for
at least one other package [0].

Install everything under ${EMACS_SITE_LISPDIR}/vterm/ instead. This
keeps the port self-contained, no longer pollutes a directory other
ports probe, and matches ELPA package's layout.

[0]
math/ess auto-detects ess-etc-directory by probing paths relative to its
own lisp directory, one of which is ../etc. With site-lisp/etc present,
ESS selected it instead of the correct share/emacs/etc/ess, so M-x R
failed to load ESSR with "cannot open the connection".

Approved by: maintainer timeout
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-ports/pull/568