added function get_free_mem_pages() that returns the current amount of free memory pages

needed for delay_loop_for_low_mem_systems() in etc/inc/pfsense-utils.inc
added a function for delaying the startup till the RAM usage is stabilized

usefull for low memory systems as often processes take a lot of memory during starting
but release it again later. So if to many processes are started at the same time
random ones are running into out_of_swap_space and get killed.
This function waits waits till the memory usage does not change anymore or a timeout is reached
delay_loop_for_low_mem_systems(integer $max_delay_sec, integer $min_memory_stabilized_sec)
Added Settings to configure the delaed startup on low mem systems

1. Maximum delay time for delay_loop_for_low_mem_systems()
2. Memory stabilized time for delay_loop_for_low_mem_systems()
3. min. memory for immediate webGUI start (if the system has less memory the start of the WebGUI will be delayed)
added delay_loop_for_low_mem_systems(); on OpenVPN client and server starts
as the PHP fork eats a lot of memory during start of OpenVPN but relases it again when the start is finished.
When all Servers/Clients are started at the same time low memory systems can run into problems as processes receive out_of_swap_space message and get killed. On our system it mostly happens during OpenVPN starts
Update an existing cron entry for pppoe periodic resets

The array variable name was incorrect in the test, so the existing cron entry was not being matched. Fixes #3192
Fix update URL so the -RELEASE version looks at the stable updates URL by default rather than the snapshots server.
Fix didn't help -- backing this out and the change that made it necessary. Revert "Correctly check the secondary/primary parameter setting on dhcp failover configuration"

This reverts commit 24670866827b4e2d7a4a05baaf6d09ee377ce7cb.
This broke correct detection of primary/secondary -- the person in that thread may have had some other config issue, but this broke working/valid configurations. Revert "Correct check to match the right vip based on configured ip. Reported-by: http://forum.pfsense.org/index.php/topic,66234.0.html"

This reverts commit 25f9f3322863eae12f705137d8d414643d7b853f.
No need to treat PHP errors as a crash on -RELEASE for now.
Merge branch 'RELENG_2_1' of https://github.com/pfsense/pfsense into RELENG_2_1