pid1: initialize TERM environment variable correctly

pid1: initialize TERM environment variable correctly

When systemd is started by the kernel, the kernel set the TERM
environment variable unconditionnally to "linux" no matter the console
device used. This might be an issue for dumb devices with no colors
support.

This patch uses default_term_for_tty() for getting a more accurate
value. But it makes sure to keep the user preferences (if any) which
might be passed via the kernel command line. For that purpose /proc
should be mounted.