Having max-jobs = 32 ($NIX_USER_COUNT is hardcoded to that value) may
severely overload the machine. The nix.conf(5) manual page says max-jobs
defaults to 1, so let's use that value.

NOTE: Both max-jobs and cores are now being set to their default value,
so they can be removed alltogether.

Hardcoding the "max-jobs" and "cores" settings in nix.conf at install
time, to the same value as Nix' built-in default, makes little sense to
me.

Minor typo

In a daemon-based Nix setup, some options cannot be overridden by a
client unless the client's user is considered trusted.

Currently, if an untrusted user tries to override one of those
options, we are silently ignoring it.

This can be pretty confusing in certain situations.

e.g. a user thinks he disabled the sandbox when in reality he did not.

We are now sending a warning message letting know the user some options
have been ignored.

Related to #1761.

…ntrusted

Daemon: warn when an untrusted user cannot override a setting

(cherry picked from commit c38c726)

This allows many programs (e.g. gcc, clang, cmake) to print colorized
log output (assuming $TERM is set to a value like "xterm").

There are other ways to get colors, in particular setting
CLICOLOR_FORCE, but they're less widely supported and can break
programs that parse tool output.

install-multi-user: reduce max-jobs from 32 to 1

Run builds in a pseudo-terminal