cryptohome: Add new platform functions

This is first in a series of CLs to add opencryptoki initialization to cryptohome.
This ones adds new platform functions for
1) changing file/path ownership recursively.
2) creating symlinks.
3) getting group id on a path.
4) executing a binary as a specific effective uid/gid.

(Original patchset by fes@chromium.org. This ones fixes some
minor bugs and contains documentation fixes.)

Change-Id: I424ca11fcb74ca5d3b040dccf0cf82ed8da766cc

BUG=chromium-os:12295
TEST=emerge-x86-mario chromeos-base/chromeos-cryptohome

Review URL: http://codereview.chromium.org/6729012
3 files changed