Improve var names in get_memory

realmem is the amount of actual (real) memory installed - the size of the RAM card - e.g. 256MB
physmem is the amount of memory available to FreeBSD after BIOS, video... has stolen some of realmem.
The variable names currently used are not very helpful for code readability. This standardises them. No functional change here.
Use updated get_memory var names

The value of minimum_ram_warning is designed to be compared to hw.physmem - so do that. Usse the appropriate physmem or realmem value in each place.
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.
Use new names for get_memory parameters
Use physmem and realmem from get_memory() in the appropriate places
Merge pull request #785 from phil-davis/master

Tidy up use of hw.physmem and hw.realmem from get_memory()