checkfc: add support for comparing two file_contexts files.

Extend checkfc to support comparing two file_contexts or
file_contexts.bin files.  This is for use by the CTS
SELinuxHostTest to compare the AOSP general_file_contexts
with the device file_contexts.bin file.

Depends on I0fe63e0c7f11ae067b5aac2f468f7842e5d76986.

Change-Id: I2fff2f8cf87690a76219ddf4cf38939650f34782
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>