Add more of the old .bzrignore to .gitignore.

Plus a few more .gitignore improvements.
* .gitignore: Add copyright notice, since it's big enough.
Put exceptions immediately after the patterns they're exceptions
to, to make them easier to follow.
Give four exceptions for the Makefile pattern.
Ignore /confdefs.h, test/indent/*.new, TAGS, GPATH, GSYMS, GRTAGS,
GTAGS, ID, *.exe, some nextstep-related files,
jisx2131-filter, *.orig, *.rej, etc/emacs.tmpdesktop, *.in-h
_* (except for build-aux/snippet/_Noreturn.h), /bin/, /BIN/,
/data/, etc/icons/, lib/cxxdefs.h, lib/SYS/, /libexec/, /lock/,
/README.W32, /share/, /site-lisp/, src/gdb.ini, /var/.
Ignore /configure.lineno, since POSIX requires
LINENO only with the User Portability Utilities option.
Ignore *cust-load.el and *loaddefs.el only under lisp.
Ignore core files of various flavors.
Do not ignore etc/refcards/gnus-logo.pdf.