BUG/MEDIUM: auth: fix segfault with http-auth and a configuration with an unknown encryption algorithm

Grégoire Morpain reported a segfault when a secured password is used for http
authentication. It was caused by the use of an unsupported encryption algorithm
with libcrypto. In this case, crypt() returns a NULL pointer.

The fix should be backported to 1.4 and 1.5.
BUG/MEDIUM: config: userlists should ensure that encrypted passwords are supported

When an unknown encryption algorithm is used in userlists or the password is
not pasted correctly in the configuration, http authentication silently fails.

An initial check is now performed during the configuration parsing, in order to
verify that the encrypted password is supported. An unsupported password will
fail with a fatal error.

This patch should be backported to 1.4 and 1.5.