Added support for the shadow authentication framework instead of PAM.
Enable it by passing --with-authfw=shadow to configure.

This is done by splitting the polkitagenthelper source into separate
parts, one that does auth with PAM, and another that does auth with
shadow, sharing functions where appropriate.

Also, all PAM-dependendent code in all other files has been #ifdef'd.
The only affected file is src/programs/pkexec.c

Signed-off-by: David Zeuthen <davidz@redhat.com>