ctags: Support writing to stdout instead of a file.

[des]

Summary

• 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.

[debdrup]

mdoc(7) changes look good to me, can't speak to the code.