maintenance(systemd): support the Windows Subsystem for Linux

maintenance(systemd): support the Windows Subsystem for Linux

When running in the Windows Subsystem for Linux (WSL), it is usually
necessary to use the Git Credential Manager for authentication when
performing the background fetches.

This requires interoperability between the Windows Subsystem for Linux
and the Windows host to work, which uses so-called vsocks, i.e. sockets
intended for communcations between virtual machines and the host they
are running on.

However, when Git is configured to run background maintenance via
`systemd`, the address families available to those maintenance processes
are restricted, and did not include `AF_VSOCK`. This leads to problems
e.g. when a background fetch tries to access github.com:

	systemd[437]: Starting Optimize Git repositories data...
	git[747387]: WSL (747387) ERROR: UtilBindVsockAnyPort:285: socket failed 97
	git[747381]: fatal: could not read Username for 'https://github.com': No such device or address
	git[747381]: error: failed to prefetch remotes
	git[747381]: error: task 'prefetch' failed
	systemd[437]: git-maintenance@hourly.service: Main process exited, code=exited, status=1/FAILURE
	systemd[437]: git-maintenance@hourly.service: Failed with result 'exit-code'.
	systemd[437]: Failed to start Optimize Git repositories data.

Address this (pun intended) by adding the `AF_VSOCK` address family to
the allow list.

This fixes https://github.com/microsoft/git/issues/604.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>