Adds two functions to librc, rc_set_user() and rc_is_user(). The latter
just queries if the former was called. The former instantiates paths for
locations that make sense for user services:

Script dirs get set to $XDG_CONFIG_HOME/openrc/{init.d,conf.d}, and
variations of the system sysconf dirs with "user.d/" as a suffix.

Runlevel dir is set to $XDG_CONFIG_HOME/openrc/runlevels, and the svcdir
is set to $XDG_RUNTIME_DIR/openrc.

XDG_CONFIG_HOME falls back to $HOME/.config should it be unset.
XDG_RUNTIME_DIR has no fallback, and thus required for user services.

Add a new function to librc, `rc_usrconfdir()`, whick returns the
location where user-made rc.conf exists.

The user-made configuration is loaded first over the global config, so
it takes priority when resolving variables, thus "overriding" global
configs.

Those variables are required for user scripts to work properly.

…ons.

Some services might expect to be in home, and may behave unexpectedly
for the user, e.g. any program started via dbus, and this matches
systemd-user behaviour.

Some binaries required to have their `atexit(cleanup)` calls moved to
after argument parsing, since setting user mode also adds an atexit
call, and cleaning up user paths should only happen after the cleanup of
the application is done, thus needs to be set first.

We need to handle --user before calling any functions that interact with
librc.