proxy.cgi: Fixes bug12755 - proxy auth problem with password longer than 8 chars

proxy.cgi: Fixes bug12755 - proxy auth problem with password longer than 8 chars

- This makes the proxy local password management the same between chpasswd.cgi and
   proxy.cgi
- Tested out on my vm testbed and was able to create and modify users and their passwords
   in the proxy.cgi page or modify a password for a specified user on the chpasswd.cgi
   page. This all happened successfully and was confirmed by testing out the local
   authentication.

Fixes: bug12755
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>