[lldb] [lldb-gdbserver] Unify listen/connect code to use ConnectionFileDescriptor

Unify the listen and connect code inside lldb-server to use
ConnectionFileDescriptor uniformly rather than a mix of it and Acceptor.
This involves:

- adding a function to map legacy values of host:port parameter
  (including legacy server URLs) into CFD-style URLs

- adding a callback to return "local socket id" (i.e. UNIX socket path
  or TCP port number) between listen() and accept() calls in CFD

- adding a "unix-abstract-accept" scheme to CFD

As an additional advantage, this permits lldb-server to accept any URL
known to CFD including the new serial:// scheme.  Effectively,
lldb-server can now listen on the serial port.  Tests for connecting
over a pty are added to test that.

Differential Revision: https://reviews.llvm.org/D111964