runtime: add throughput counter

throughput_avg is calculated with a wall clock timer over all items produced.
digital: fix false trigger bug in correlate_access_code_tag, add QA code.
grc: fix xterm setting check
grc: catch EOFError on fg exit
Merge remote-tracking branch 'gnuradio-wg-grc/maint_grcwg' into maint
Merge remote-tracking branch 'bistromath/pkt' into maint
Merge remote-tracking branch 'nwest/throughput'
Merge branch 'maint'