Improve var names in get_memory

Backport from master
Use updated get_memory var names 

Backport to 2.1
Use hw.physmem when calculating pfsense_default_state_size

hw.physmem is the actual amount of memory that FreeBSD/pfSense can get its hands on, so use this for the calculation.
Backport to 2.1
Use new names for get_memory parameters
Use physmem and realmem from get_memory() in the appropriate places

Backport to 2.1
Merge pull request #794 from phil-davis/RELENG_2_1

Backport get_memory changes to 2.1