core: prefs updated to handle cases of env var delared properly.
ctrlport: adding a config param to enable/disable controlport, default=disabled.

Also updates QA code for ctrlport to turn controlport on via env var.
ctrlport: using prefs files to configure ctrlport.

Prefs file points to ctrlport config file and uses it to set the properties.
core: use prefs file to enable/disable ctrlport export of perf. counters.
docs: adding an exclude rule for gr-modtool to avoid conflicts.
core: prefs setup looks for a local .gnuradio/config.conf to use.

Order of precedence:
env. vars -> .gnuradio/config.conf -> ${prefix}/etc/gnuradio/conf.d/*
docs: adding an exclude rule for gr-modtool to avoid conflicts.
docs: correcting gr-modtool exclude path for master dir format.
core: prefs setup looks for a local .gnuradio/config.conf to use.

Order of precedence:
env. vars -> .gnuradio/config.conf -> ${prefix}/etc/gnuradio/conf.d/*
Merge branch 'master' into next

Conflicts:
	docs/doxygen/Doxyfile.in