Fix list miscounting in sg_set_valid_filesystems.

Previously this counted the NULL entry that terminated the list -- so
the length was set to be one too high, and reading the list back with
sg_get_valid_filesystems would produce a spurious NULL at the end.
Update the FSF's address in the licensing notices.

The FSF moved several years ago; this updates all the mentions of their
address in the GPL/LGPL copies (for which I've just taken the latest
plain-text copy of the GPLv2 and LGPLv2.1 from the FSF web site) and the
licensing headers in our files.