[FS#609] procd: implement runlevel 1

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Stijn Tintel (stintel) 

Attached to Project - LEDE Project
Summary - procd: implement runlevel 1
Task Type - Feature Request
Category - Base system
Status - New
Assigned To - 
Operating System - All
Severity - Low
Priority - Medium
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Sysupgrade currently kills processes with SIGKILL if they don't shut down within 3 seconds after the SIGTERM signal. Some processes (e.g. domoticz) can take longer than 3 seconds to properly shut down. As this can potentially corrupt the sqlite database, it should be avoided.

While talking about this on IRC, it was suggested we should implement an "init 1" equivalent in procd. Adding this ticket as a reminder.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=609