The option was introduced in 6bb0d7b456
without documentation and intended use. The recent change
f6ae2094fe propagated this option to the
generated service files, which is resulting in unexpected results, when
a user decided to set only `sshd_config_file` for the second sshd
service causing the service file points to the system-wide
configuration file.
This is an attempt to fix this by introducing some heuristics to guess
if the user wants to set up second drop-in directory (ending with .d)
or create a standalone configuration file.
Fixes: #280
