uip/folder.c: Rewrite plural test to common form.

uip/folder.c: Rewrite plural test to common form.

The common form is a choice between "" and "s".  This one occurrence was
using " " and "s" so either output was a fixed width.  Alter the
printf(3) format string from `%s' to `%1s' to provide that instead.
Allows the new code to be altered by an upcoming `plural' change.