checkseapp: Detect duplicate entries within seapp_contexts.

Presently it only detects complete duplicates if you specify -s (strict),
which is not used in the external/sepolicy Makefile, and it allows
overriding earlier entries that have the same input selectors (e.g.
user=, seinfo=) with different values for the output selectors (e.g.
domain=, type=).  Thus, a device/<vendor>/<board>/sepolicy/seapp_contexts
file can override the external/sepolicy definitions, and even a single
seapp_contexts file can contain duplicated or conflicting definitions.

Make it always check strictly, and prohibit either duplicates on the
input selectors (i.e. overrides) or complete duplicates (redundant).

Change-Id: Id1e38133cbe31b796253101cfe3b111d1826bc8c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>