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At present, the label_file backend expects to be provided the path
to the text file_contexts file and always appends the .bin suffix
when checking for the binary file_contexts.bin file.  If one
attempts to directly specify the path to a file_contexts.bin file
to selabel_open(), it will fail as the code will append a second
.bin suffix to it.  Check to see if the file path already has a .bin
suffix and do not append it in that case.

Change-Id: Id560d093440a2aba99cef28c20133b35feebf950
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>