Feature: Allows "-" as filename for the `--log` option.

When (e.g. for debugging purpose) the rest server is invoked like

	sudo -u restic rest-server … --log /dev/stdout

it tries to open `/dev/stdout` (O_CREATE, O_WRITE, O_APPEND). This
operation fails, as in the above invocation, `/dev/stdout` is owned by
the caller (here: root) and only writable for the caller. Subprocesses
get just the filedescriptor. Using `/proc/self/fd/1` didn't work either,
for the same reasons.

https://github.com/restic/rest-server/pull/217