Support Go language in 'etags'

Support Go language in 'etags'

* lib-src/etags.c <Ruby_help>: Fix documentation of Ruby tags.
<Go_help>: New help.
<Go_suffixes>: New variable.
(Go_functions): New function.
<lang_names>: Add entry for Go.  (Bug#22370)

* doc/emacs/maintaining.texi (Tag Syntax): Document Go support.
* doc/man/etags.1: Mention Go support.

* etc/NEWS: Mention Go support.

* test/etags/go-src/test.go:
* test/etags/go-src/test1.go: New test files.
* test/etags/Makefile (GOSRC): New variable.
(SRCS): Add $(GOSRC).
* 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 addition of Go tests.