conf-parser: let's explicitly deprecate .include in unit files

.include lines are already deprecated somewhat, and for example
explicitly not mentioned in the documentation for this reason. Let's get
one step further and generatea warning when we encounter them (but still
process them).

Why are they deprecated? Because they are semantically awful — they
complicate stat() based mtime checks for configuration files and they
allow arbitrary loops we currently have zero protection against and
really shouldn't have to have.