Create a struct to hold pointers to all functions needed by plugins.
This eliminates the need for compiling with `-rdynamic`, and fixes
the plugins linking on Cygwin.
Fix scope of memset from 35ff7c6.
Fix license text in multiple files.
Revert the following 3 commits related to Makefile:

	cef930d: Makefile: Follow Thermi's commit to fix DESTDIR.
	708eebc: Makefile: Add support for DESTDIR.
	4fd35d7: Revert "Fix support for DESTDIR."
Don't exit with an error if the data directory table is empty.
Possible fix for issue #78. Thank you Katja Hahn!