discover: Update env vars at init and suppress LVM warnings

Environment variables are not platform-specific so move
set_proxy_variables to device-handler and call it at handler init.

At the same time set LVM_SUPPRESS_FD_WARNINGS to ignore the "file
descriptor leaked" warnings when calling LVM-utilities, since we must
keep some file descriptors open in lib/process.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
(cherry picked from commit 3132110c344c60bbe648b5e1b0473bdec101d111)