core: add support for StandardInputFile= and friends

core: add support for StandardInputFile= and friends

These new settings permit specifiying arbitrary paths as
stdin/stdout/stderr locations. We try to open/create them as necessary.
Some special magic is applied:

1) if the same path is specified for both input and output/stderr, we'll
   open it only once O_RDWR, and duplicate them fd instead.

2) If we an AF_UNIX socket path is specified, we'll connect() to it,
   rather than open() it. This allows invoking systemd services with
   stdin/stdout/stderr connected to arbitrary foreign service sockets.

Fixes: #3991