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

hw.realmem is the real (actual) amount of memory in the system - the size of the memory card, e.g. 256 MBytes
hw.physmem is the amount of memory that  FreeBSD (pfSense) can have, after BIOS, video... steals some of the memory - e.g. on Alix it show about 235 MBytes.
(Note: these are stored as bytes by the system. get_memory converts them to MBytes when returning them)
$g['minimum_ram_warning'] is set to 101, and is designed to be compared to hw.physmem, but sometimes gets compared to hw.realmem
This pull request cleans up the naming to make the code easier to understand (IMHO), and also does the appropriate comparison with physmem or realmem in each place.