Add seastar signal api

[tomershafir]

fixes: https://github.com/scylladb/seastar/issues/261.

The PR adds an object based interface for signals on seastar namespace.

• Adds signal.hh header implemented by reactor.cc.
• Adds doc/signal.md.
• Deprecates reactor::handle_signal.
• Supports C++20 modules.

Future possibilities:

• Add signal handler unset logic, and unset signal handler on destructor.
• Consider to add a reliable test case for once (should require reactor changes to expose an indicator).