core: add new UnsetEnvironment= setting for unit files

With this setting we can explicitly unset specific variables for
processes of a unit, as last step of assembling the environment block
for them. This is useful to fix #6407.

While we are at it, greatly expand the documentation on how the
environment block for forked off processes is assembled.

src/core/execute.h

@@ -133,6 +133,7 @@ struct ExecContext {
         char **environment;
         char **environment_files;
         char **pass_environment;
+        char **unset_environment;
 
         struct rlimit *rlimit[_RLIMIT_MAX];
         char *working_directory, *root_directory, *root_image;