applets/install: don't try to install nothing

applets/install: don't try to install nothing

Commit 952d5a6024e7 (applets/install: accept more than one install
option) changed the way we handle install options: before that commit, a
missing install type would mean to install nothing; after, we would
iterate over options, so we would never notice there was a mising
option.

Fix that by introducing an explicit --none option to specify to install
nothing.

Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>