Introduce '## ' as internal comment prefix in .in files and filter out a comment (#5289)

Sometimes we have comments which don't make sense outside of the systemd
codebase, so let's filter them out from the user-visible files.

Fixes #5286.

Makefile.am

@@ -6244,6 +6244,7 @@ substitutions = \
 SED_PROCESS = \
 	$(AM_V_GEN)$(MKDIR_P) $(dir $@) && \
 	$(SED) $(subst '|,-e 's|@,$(subst =,\@|,$(subst |',|g',$(substitutions)))) \
+                -e '/^\#\# /d' \
 		< $< > $@
 
 units/%: units/%.in

units/systemd-nspawn@.service.in

@@ -22,9 +22,9 @@ Slice=machine.slice
 Delegate=yes
 TasksMax=16384
 
-# Enforce a strict device policy, similar to the one nspawn configures
-# when it allocates its own scope unit. Make sure to keep these
-# policies in sync if you change them!
+## Enforce a strict device policy, similar to the one nspawn configures
+## when it allocates its own scope unit. Make sure to keep these
+## policies in sync if you change them!
 DevicePolicy=closed
 DeviceAllow=/dev/net/tun rwm
 DeviceAllow=char-pts rw