Add support for midipix #42
Description
midipix is a new POSIX-on-Windows effort I just learned of, distinct from the Cygwin/MSYS/MSYS2 family. Conceptually, ssh-pageant should be portable to such an environment as long as it supports UNIX sockets and the WinAPI calls for talking to Pageant.
(I'm also assuming they'll have openssh etc. to even want key management in the first place.)