[System.Runtime.Caching] Added Mono config variables.
Variables __MonoEmulateOneCPU and __MonoTimerPeriod are used by some tests.
[System.Runtime.Caching] Removed UnsafeNativeMethods calls on Mono.
 - MEMORYSTATUSEX usages were replaced with performance counters
   "Total Physical Memory and "Available Physical Memory"

 - SafeRegistryHandle.ReleaseHandle is only called on code that is ignored on Mono.

 - Mono is using AppDomain.FriendlyName directly instead of UnsafeNativeMethods.GetModuleFileName.
Merge pull request #3 from esdrubal/caching

System.Runtime.Caching integration changes
Adding reference source for System.Net
Merge branch 'master' into mono
Use the DISABLE_CAS_USE macro instead of MOBILE to disable CAS

This augments commit 7dc4e513eafd3fb30d34111948ef14c981264463
[TPL] Add mono internal API support