Improve Ruby support in 'etags'

Improve Ruby support in 'etags'

* lib-src/etags.c (Ruby_functions): Tag constants.  Don't tag
singleton classes.  Remove class qualifiers from tags generated
for method and constant names.  (Bug#22241)

* doc/emacs/maintaining.texi (Tag Syntax): Mention that constants
are tagged by etags in Ruby.

* etc/NEWS: Mention that constants are tagged by etags in Ruby.

* test/etags/ruby-src/test1.ruby: Add more tests.
* test/etags/ETAGS.good_1:
* test/etags/ETAGS.good_2:
* test/etags/ETAGS.good_3:
* test/etags/ETAGS.good_4:
* test/etags/ETAGS.good_5:
* test/etags/ETAGS.good_6:
* test/etags/CTAGS.good: Adapt to the changes in etags and in Ruby
tests.