su: add --whitelist-environment

su: add --whitelist-environment

* usable with --login to whitelist specified environment variables

* the list is ignored for the core variables like HOME, SHELL, USER,
  LOGNAME and PATH (su --login always resets these variables)

Note that su(1) requires password and after successful authentication
user has full control over the session, so he can set arbitrary
environment variables. The whitelist makes things more user friendly
only.

The patch removes unnecessary optimization when allocate environ[]. It
seems better to keep all in glibc hands and just reset the environment
array only.

Addresses: https://github.com/karelzak/util-linux/issues/221
Signed-off-by: Karel Zak <kzak@redhat.com>