make it easier to find out how a var was `set`

Hello,

I ran into a problem when my home folder was moved and the default environment variable values in fishd. were still pointing to the old directories.
I wasn't aware that the file overwrote the defaults and by calling `fish --debug-level=5` I still wasn't able to ascertain if the default values had been hard-coded, compiled using #ifdefs , etc.

Only a grep of the faulty values in all files with "fish" in their name (`grep yada $(find -name "*fish*" ~)`) gave me a hint and `rm ~/.config/fish/fishd.*` gave me resolve.

**TL;DR Please adding logging to the cpp-function called when calling `set` in fish.**

Cheers!