Added system.IsSafe() for OSX

Added an implementation of system.IsSafe() for OSX; it checks that the output of sysctl.vmusage contains "(encrypted)".

Btw, according to the manpage for dynamic_pager, although encrypted swap is on by default for portable computers and off by default for other Macs. As I don't know anyone who has a Mac desktop, I can't confirm whether this is accurate. (The manpage may be outdated; I don't believe that the preferences plist it references exists on modern OSX.)