This makes using the nixos channel work out of the box with the new
Nix commands. For example:

$ nix run nixos.firefox -c firefox

Fixes #46536

khelpcenter needs meinproc5 to work properly. Hopefully doesn’t effect
closure sizes too much - kdoctools is rather small.

Fixes #46539

/cc @ttuegel

Without this the graphical installer has no way to open the manual.
You can fix it yourself by installing any HTML browser but this might
be unfamiliar to users new to NixOS and without any other way to open
the manual. The downside is it will also increase download sizes.

Fixes #46537

(cherry picked from commit f1f2c2d3fc74d52f445f08f0f0410d1be586532b)

(cherry picked from commit 2481b366f6ea964889d1010e4159055cef7b5b95)

(cherry picked from commit 2de9705b9fcaa3650ea85a9f5e2aee834948fc8f)

Also remove haddock-library == 1.4.4 since it's not needed anymore

(cherry picked from commit 77c73cc8eb29da2154d2e9087d7efd7fd9af10f9)

/run/rmilter is set by systemd, and have root:root ownership, which
prevent pid file to write.

This fix suggested to be promoted to 18.09 branch.
(Although rmilter itself is deprecated, and I plan to remove it, after
18.09 would be released)

(cherry picked from commit 08f2664)

(cherry picked from commit 868040e)

environment.sessionVariables cannot refer to the values of env vars,
and as a result this has caused problems in a variety of scenarios.

One use for these is that they're injected into /etc/profile,
elewhere these are used to populate an 'envfile' for pam
(`pam 5 pam_env.conf`) which mentions use of HOME being
potentially problematic.

Anyway if the goal is to make things easier for users,
simply do the NIX_PATH modification as extraInit.

This fixes the annoying problems generated by the current approach
(#40165 and others) while hopefully serving the original goal.

One way to check if things are borked is to try:

$ sudo env | grep NIX_PATH

Which (before this change) prints NIX_PATH variable with
an unexpanded $HOME in the value.

-------

This does mean the following won't contain user channels for 'will':
$ sudo -u will nix-instantiate --eval -E builtins.nixPath

However AFAICT currently they won't be present either,
due to unescaped $HOME.  Unsure if similar situation for other users
of sessionVariables (not sudo) work with current situation
(if they exist they will regress after this change AFAIK).

(cherry picked from commit fa67ca15430e96b9d165972552403be6b3f4817e)