Use random temporary data directory

See merge request !29

Run mms-engine as current user

See merge request !28

MMS engine needs access to MMS settings stored in DConf.

Unit tests fail with libwspcodec older than 2.2.3

Add sendMessageFd D-Bus API

See merge request !27

It allows to pass attachments to mms-engine as file descriptors rather
than file names. That way, permissions on the attachment files won't
have to be wide open to everyone.

Convert text parts to utf-8

See merge request !26

Those operators who convert text attachments to utf-8, seem to
only understand utf-8 as a charset. If they see utf-8, they do
nothing; if they see anything else, they interpret that as us-ascii
and convert it to utf-8 (which typically messes things up).

Therefore, we need to convert text parts into utf-8 and specify
utf-8 charset in the attachment part header. That way it should
always work.

This automatic utf-8 conversion can be turned off with a command
line option, or via config file. By default, conversion is on.

Run mms-engine under radio account

See merge request !25

ofono is running under radio, it must be guaranteed to be present.

Logging over D-Bus

See merge request !23

Allows sufficiently privileged client to stop the service in case if
mms-engine needs to be restarted under a different user account.

Now that it's a particularly useful feature but somehow it was
missed, let it be there for completeness sake.

Signed-off-by: Matti Kosola <matti.kosola@jolla.com>

[aarch64] Fix location of systemd unit files. Contributes to JB#50027

See merge request !24