The varlink spec supports protocol upgrades and they are very
useful to e.g. transfer binary data directly via varlink. So
far sd-varlink was not supporting this.
This commit adds a new public sd_varlink_call_and_upgrade()
that sends a method call, waits for the reply, then steals
the connection fds for raw I/O. It returns separate input_fd
and output_fd to support both bidirectional sockets and pipe
pairs.
A helper is extracted and shared between sd_varlink_call_full()
and sd_varlink_call_and_upgrade(). A new `protocol_upgrade`
bool in `struct sd_varlink` ensures that on a protocol upgrade
request we only exactly read the varlink protocol bytes and
leave anything beyond that to the caller that speaks the upgraded
protocol.
Note that this is the client side of the library implementation
only for now. The server side needs work but this is already
useful as it allows to talk to varlink servers that speak protocol
upgrades (like the rust implemenations of varlink).
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