MFC r306375,r307802:

r306375 (by emaste):

Add a WITHOUT_DIALOG src.conf(5) knob

It also turns off dependencies (bsdinstall, bsdconfig, dpv, tzsetup).

r307802 (by bapt):

Fix build of tzsetup when WITHOUT_DIALOG is set

Hide dialog specific code behind HAVE_DIALOG. It allows to build a stripped
down version (missing the dialog UI) but perfectly function tzsetup when
world is built WITHOUT_DIALOG

Reorganise a bit the code to limit the number of blocks under HAVE_DIALOG