When using `etags-regen-mode` with Universal Ctags, I receive the following
error message when appending to the TAGS file: `ctags: -o option may not
follow a file name`.

NOTE: This error only occurs when appending to the TAGS file, not when
initially generating the file.

The attached patch file fixed the error for me. I tested the patch using
both `etags` and Universal `ctags`.

Information:
ctags version: Universal Ctags 6.1.0
OS: macOS 14.2.1