Add a "mode" metadata key to the unix listener so users can set the
file permissions on the unix domain socket after it is created. This
allows programs running under a different user/group to connect to the
socket without requiring a global umask override.
Values accept Go-style octal (0660), Go literal (0o660), or decimal
(432), both via URL query params and YAML config files.
Fixesgo-gost/gost#878