Add GH_SUBDIR, automatically moves a secondary distfile to the right place inside ${WRKSRC}.

[mat]

Summary

Before:

GH_TUPLE=	Regaddi:Chart.js:f13f99b:chart_js \
		FVANCOP:ChartNew.js:77e7f87:chartnew_js
post-extract:
	@${RMDIR} ${WRKSRC}/database ${WRKSRC}/3rd/Chart.js
	@${MV} ${WRKSRC_database} ${WRKSRC}/database
	@${MV} ${WRKSRC_chart_js} ${WRKSRC}/3rd/Chart.js

After:

GH_TUPLE=	Regaddi:Chart.js:f13f99b:chart_js/3rd/Chart.js \
		FVANCOP:ChartNew.js:77e7f87:chartnew_js/3rd/ChartNew.js

It also works if not using GH_TUPLE but the regular GH_ACCOUNT/PROJECT/TAGNAME variables:

GH_SUBDIR=	3rd/Chart.js:chart_js 3rd/ChartNew.js:chartnew_js

[mat]

• Try to make sure where we move things to exists.
• Provide a way to have the :DEFAULT GH_SUBDIR work.

[mat]

• Provide a way to have the :DEFAULT GH_SUBDIR work.

[mat]

• Convert other ports.
• Remove the / in front of the GH_SUBDIR entries, it was ugly.
• A couple more ports.