Using parse_birthdate() whenever changing user's birthdate field via Baja/JS

This means that when writing user.birthdate (in JS) for example, we will
always save the field in the proper (YYYYMMDD) format. This fixes issue #855.

Remove use of SM_EURODATE from userdat.c.

This commit includes new @-code MINSPACE (minimum diskspace required to allow
uploads) too (unrelated change).