Improve neverallow error messages and allow disabling them on userdebug builds.

This patch adds a flag that can be used to ignore neverallow rules.
By adding
SELINUX_IGNORE_NEVERALLOWS := true
into the BoardConfig.mk file, neverallow violations will be ignored
silently.  This flag can only be enabled on userdebug and eng builds.

Users of this flag should be very careful.  Since it does not work on
user builds, it must be disabled to pass CTS, and enabling it for
too long could hide issues that need to be addressed.

As a happy side effect, this patch should also improve the error
messages when violating a neverallow rules.  Specifically, the file
and line number should be correct.

Bug: 70950899
Bug: 33960443
Test: Built walleye-{user,eng} with and without this new option and
a neverallow violation.  Built policy for all targets.

Change-Id: Id0d65123cdd230d6b90faa6bb460d544054bb906