With the new protocol upgrade support in varlinkctl client we can
now do the equivalent for the server side. This commit adds a new
`serve` verb that will serve any command that speaks stdin/stdout
via varlink and its protocol upgrade feature. This is the
"inetd for varlink".
This is useful for various reasons:
1. Allows to e.g. provide a heavily sandboxed io.myorg.xz.Decompress
varlink endpoint, c.f. xz CVE-2024-3094)
2. Allow sftp over varlink which is quite useful with the
varlink-http-bridge (that has more flexible auth mechanism than
plain sftp).
3. Makes testing the varlinkctl client protocol upgrade simpler.
4. Because we can.
Michael Vogt
34f29079fd