Add library functions for HTTP client authentication

Add library functions for HTTP client authentication

These functions can be used by HTTP clients to authenticate against
HTTP servers using the Basic and Digest algorithms as described in
RFC2617.

Most of the code is taken from modules/access/http.c, although it
includes modifications to make it work as library functions and to
fix some issues. The HTTP access module can be converted at a
later point, but there's still some stuff needing cleanup first.

These functions will be used for the Remote Audio Output Protocol
plugin to authenticate VLC against RAOP-compatible devices if the
user enabled password protection.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>