gmessages: add a new log writer for new-style daemon logging
New-style daemons controlled by systemd can use the straightforward way of logging to stderr. Log levels can be specified by prefixing each line with headers just like the linux kernel's printk specifies e.g.: '<4>' for WARNING level.
This patch adds a new log writer that automatically translates the GLib log levels to their corresponding new-style headers and prints all messages to stderr.
Signed-off-by: Bartosz Golaszewski firstname.lastname@example.org