Allow JS 'user.editor' and '.shell' to be set for non-users (e.g. user #0)

The request from Nightfox and Accession via DOVE-Net was to be able to set
a user's external editor even if there's no user logged-in.

These 2 user class properties in the JS object model were a bit special in
that they *only* wrote to the user database and did not immediately modify
the in-memory copy of the user_t structure, depending on the re-reading of
the user.dat/tab file to re-populate the current user_t structure when needed.
This didn't work if the current user is user #0 (no user).

So, set the current user_t.xedit and user_t.shell accordingly whenever those
JS properties are assigned a value (a string, the appropriate internal code).