session: deny access if password login is disabled

session: deny access if password login is disabled

The special characters '!' and '*' in /etc/shadow are
used to disable password login for a specific account.

The character 'x' has no special meaning, but should not
be interpreted as an empty password.

However, rpcd did treat these special characters like no
password was set, which allows access even though the account
is disabled.

By removing the additional checks for these characters, the
encrypted password string is passed to crypt, which returns NULL
if the salt has an invalid format and therefore access is denied.

Fixes: FS#2634

Signed-off-by: Fabian Bläse <fabian@blaese.de>