[minijail] Add the ability to set capabilities from the command line

make `minijail --use-capabilities [bitmask]` set the effective and
bounding capability sets of the jailed process.

BUG=14302
TEST=unit tests

Change-Id: I66eb18145fb7ed466c4629a7cb32fe12deea2c0f

R=wad@chromium.org

Review URL: http://codereview.chromium.org/6881066
6 files changed