Fixes#6043 - Reimplement UnixSocket support based on Java 16.
* Introduced new module "jetty-server-unixdomain".
It uses reflection to access the Java 16 Unix-Domain classes to keep compatibility with the other modules and the build.
* Added Jetty module with only HTTP/1.1 support for now (requires review of the modules to reuse them with various connectors).
* Updated documentation to mention UnixDomainServerConnector.
* Updated client libraries to support Unix-Domain.
* Updated PROXY protocol implementation to support Unix-Domain.
* Replaced unix.socket.tmp with better named jetty.unixdomain.dir property.
Defaulted jetty.unixdomain.dir property to system property user.home under Windows.
Simplified code that runs Unix-Domain tests.
Signed-off-by: Simone Bordet <simone.bordet@gmail.com>
Simone Bordet