Log Message:
pkg_install: Hide stdout if upgrading/refreshing.

Helps avoid problems where users see pkg_delete messages about removing
configuration files even though they are still in use and required, as
well as annoyingly repeating MESSAGE files on every upgrade or refresh.

Packages that do not have an existing version (i.e. new installs)
continue to show install script output as well as MESSAGE files, and
explicit pkg_delete operations will obviously show deinstall script
output too.

From Jonathan Perkin <jperkin@smartos.org>
via drecklypkg commit $64df139d33d2af103f51a832c2f253ff068dd743