Add SELinux MAC to debuggerd.

debuggerd allows uid-0 processes to dump tombstones or backtraces
of any process, and uid-system processes to dump backtraces of any
processes.  Restrict these operations via SELinux based on the
client context, the target process context, and the action.

Depends on I8e120d319512ff207ed22ed87cde4e0432a13dda for the
corresponding policy definitions.

Change-Id: Ib317564e54e07cc21f259e75124b762ad17c6e16
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>