msi: Correct the setting of environment variables.

msi: Correct the setting of environment variables.

Contrary to the MSDN, you can 'create only' whilst using a previous value
(ie +-variable and [~];value). This breaks the install of an application
I was debugging who uses that throughout for all their variables. In
addition whilst adding some tests I've also cleaned up invalid prefix
syntax handling and semicolon removal if the original string is empty.