digest+m_auth: Allow a domain in the digest username.

Before this patch, a domain in the digest username was allowed:

    Digest username="abc@domain", realm="domain"

but only if the domain is equal to the realm. This was introduced in
a92bf789 (Juha Heinanen, Mar. 29 2006).

The SIP spec. doesn't mention any such restriction.

This patch undoes that commit and makes sure the whole username is used
when authenticating through `pv_www_authorize` and `pv_proxy_authorize`.
That makes the following valid:

    Digest username="abc@domain", realm="something different"
Fixed bogus test over the presence of a request route. This was generating bogus warning messages to the logs, but without any negative impact over the actual processing.
Credits go to rrb3942 on GITHUB
Closes issue #162
Fix NULL pointer deref. in "acc_db_request" when "db_url" is not set
(cherry picked from commit 5d61f3789b191c04d0595aed6e1eb94392fc9e0c)