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.
(cherry picked from commit c82279c5fc70a6d12ab6efd61e6c9f536e5d4a3f)
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.
(cherry picked from commit 1a0191d2ff83d1ce7458ebf8b54640ab0a8acf23)