iwnstats: fix build with clang and allow install under /usr/local/sbin

iwnstats was not compiling because of some issues raised by the clang
compiler due to -Werror. As a tool it is not connected to world build.

Add missing field "barker_mrc" initialization in struct
iwn_sensitivity_limits for -Wmissing-field-initializers, remove unused
pointer *is on iwn_stats_*_print functions and unused variables for
-Wunused-parameter and -Wunused-variable.

The value for field "barker_mrc" of struct iwn2030_sensitivity_limits
was obtained from linux 3.2 wireless/iwlwifi driver code (iwl-2000.c:115
.barker_corr_th_min_mrc = 390).

Also set BINDIR in Makefile to make it possible to install under
/usr/local/sbin/iwnstats as it require super user.

Reviewed by: adrian
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29800