unix-socket: disallow chdir() when creating unix domain sockets
Calls to `chdir()` are dangerous in a multi-threaded context. If
`unix_stream_listen()` or `unix_stream_connect()` is given a socket
pathname that is too long to fit in a `sockaddr_un` structure, it will
`chdir()` to the parent directory of the requested socket pathname,
create the socket using a relative pathname, and then `chdir()` back.
This is not thread-safe.
Teach `unix_sockaddr_init()` to not allow calls to `chdir()` when this
flag is set.
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>