perf ui: Add ui_helpline methods

Initially this was just to be able to have a printf like method to
prepare the formatted string and then pass to newtPushHelpLine, but as
we already have for ui_progress, etc, its a step in identifying a
restricted, highlevel set of widgets we can then have implementations
for multiple widget sets (GTK, etc).

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>