Added Name parameter to the part's Content-Type header

To and From headers in M-Retrieve.conf are optional. If those are missing,
NULL strings would be passed to generated D-bus proxy which doesn't like it.

That's one of the ways to pass the name of the attachment to the recipient.
And this is what most MMS implementations are doing nowadays.

Ignore broken part headers

GSettings backend for per-SIM settings

There are too many log modules to print them all every time when help
is requested. Fine-tuning per-module log levels is a fairly rare task.

Since we know the total size of the part headers, a failure to parse them
doesn't stop us from parsing the rest of the PDU.

Broken part headers do occur in real life. One such example has been added
as a test case.

Handle more X-Mms-Response-Status values

Improved handling of expired push notifications

Also, ignore failure to parse part headers. If we know the total size of the
part headers, such a failure doesn't stop us from continuing to parse the rest
of the PDU. The purpose of this tool is not to validate the PDU but to dump as
much information as possible even if PDU is not perfectly valid.

There are quite a few files under mms-lib/test/mms_codec/data that
can be used to try this thing out. There's no need to duplicate them.

Also, deal with unknown values according to the spec.

Support for persistent per-SIM settings

This commit defines MMSSettings interface but doesn't actually implement
persistent settings. The implementation will follow as soon as it becomes
clear which kind of storage to use.

Handle X-Mms-Retrieve-Status in M-Retrieve.conf

It's one of the ways of telling us that the message has expired.

Handle X-Mms-Read-Report in M-Retrieve.conf

Better type checking

This tells us whether the sender requested a read report.

It's not being used.

Handle empty header values

Made User-Agent configurable from the command line

I suspect that some issues in the field might be related to the value
of the User-Agent header. Better have it configurable.

Limit receive task timeout by 10 minutes