SystemInformation: Fix sw_vers output processing

Commit 857bda17 (SystemInformation: Report more information, 2012-05-29)
attempted to refactor the internal CallSwVers helper to avoid duplicate
code but introduced a regression in output parsing.  Run sw_vers using
our RunProcess helper rather than popen and restore the original output
processing approach.

Change-Id: I550af5813dcad5beaec86c7c5fa1f00bcc9070b6