debianpkg: improve VTYSH_PAGER environment check

debianpkg: improve VTYSH_PAGER environment check

The current post-installation scripts for all Debian packages execute
grep 'VTYSH_PAGER=/bin/cat' to check if the VTYSH_PAGER variable is
present within /etc/environment.

While presence of that environment variable should be checked, the
current implementation does not handle this line being a comment (and
therefor not active) or the user picking a different VTYSH_PAGER than
/bin/cat.

This commit ensures that the environment variable can be freely changed
by the user, while still guaranteeing that it is present in the file
without being a comment.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>