Make only libust and libustconsumer use a signal safe usterr.h

Copy usterr.h to usterr_signal_safe.h and rewrite those parts of
usterr.h that depended on libustsnprintf. This removes the dependency
on libustsnprintf from all parts of ust except libust.

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>