Tighten unit parsing in internal values
Interval values now generate an error when the user has multiple
consecutive units or a unit without a value. Previously, it was
possible to specify multiple units consecutively which is contrary to
what the documentation allows, so it was possible to finish with
confusing interval values.
This is a follow-up of the work done in
165d581f146b.
Author: Joseph Koshakow
Reviewed-by: Jacob Champion, Gurjeet Singh, Reid Thompson
Discussion: https://postgr.es/m/CAAvxfHd-yNO+XYnUxL=GaNZ1n+eE0V-oE0+-cC1jdjdU0KS3iw@mail.gmail.com