ctags: Support writing to stdout instead of a file.

ctags: Support writing to stdout instead of a file.

* Understand "-" to mean stdout as per convention.
* Check that the output is a regular file and ignore -u if not, otherwise we might try to rm /dev/stdout.
* As a bonus, if -u was specified but the output file does not exist, proceed as if -u had not been specified instead of erroring out.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	cracauer, debdrup
Differential Revision:	https://reviews.freebsd.org/D40237

(cherry picked from commit 430d064ba5b0cb2e91a26af34c15df48d290e417)

ctags: Recognize __attribute__ in function declarations.

MFC after:	1 week
Obtained from:	NetBSD
Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D40264

(cherry picked from commit 9f35eb8f96a55aac92fc0c7d08f4b6ab0ae9bc39)

ctags: Don't undercount lines when handling // comments.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D40374

(cherry picked from commit 7ba6119e22b54c2104b1948f0e8a4157d4112958)