pr_log: a new module that provides a simple run-time controlled logging interface

A simple printf logging infra where levels are determined by the
value of the "DEBUG" environment variable.

This is loosely based on the interfaces & functionality of Linux's
printk() and pr_*() wrapper macros.

Note that the current implementation uses "<N>" prefixes (where N is a
syslog level in ascii), allowing other programs that parse log output
(like systemd's journald) to know what the priority level is.

Signed-off-by: Cody P Schafer <dev@codyps.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>