Add build and package support to minijail

Packaging:
- Adds packaging for minijail
- Adds support for building minijail in the chroot (libcap-dev)
- Fixes libcap2 versioning to be compatible with libcap-dev in the repo
- Build minijail as part of the platform packages
Code:
- Fixed an initialization check bug in minijail and unittests

Review URL: http://codereview.chromium.org/465106

build_platform_packages.sh

@@ -29,7 +29,7 @@ PLATFORM_DIR="$SRC_ROOT/platform"
 PLATFORM_DIRS="acpi assets fake_hal init installer login_manager \
                memento_softwareupdate pam_google window_manager \
                cros chrome screenlocker cryptohome \
-               monitor_reconfig"
+               monitor_reconfig minijail"
 
 THIRD_PARTY_DIR="$SRC_ROOT/third_party"
 THIRD_PARTY_PACKAGES="connman e2fsprogs/files gflags gtest \