Add cached total system memory to INFO output

There is no standard cross-platform way of obtaining
system memory info, but I found a useful function
convering all common platforms.  I removed support
for uncommon Redis platforms (windows, AIX) and left
others intact.

For more info, see:
http://nadeausoftware.com/articles/2012/09/c_c_tip_how_get_physical_memory_size_system

The system memory info is cached on startup, but some systems
may be able to change the amount of memory visible to Redis
at runtime if Redis is deployed in a VM or container.