Merge branch 'ew/daemon-socket-keepalive' into maint

When "git daemon" is run without --[init-]timeout specified, a connection from a client that silently goes offline can hang around for a long time, wasting resources. The socket-level KEEPALIVE has been enabled to allow the OS to notice such failed connections. * ew/daemon-socket-keepalive: daemon: enable SO_KEEPALIVE for all sockets