Refactor Union Station support code.

Split it into multiple files and rename classes.
Fix "mach_vm_map can't allocate region securely" warning messages on OS X Mavericks.

These were already harmless, but now we've gotten rid of them completely.

See https://code.google.com/p/phusion-passenger/issues/detail?id=1094
Rename some variables in UnionStationTest.
Send system metrics to Union Station.
Log Passenger version in the system metrics XML
'passenger-config system-metrics --xml' should end the output with a newline
Add stdin control mode to `passenger-config system-metrics`.

With this feature, the program only updates and prints metrics
every time a newline is received on stdin.
Have SystemMetricsCollector support empty newlines in /proc/stat
Move SystemMetricsCollector's swap in/out rate to from the general category to the memory category