main-func: Reduce transitive includes

Let's move some logic from _DEFINE_MAIN_FUNCTION() and other places
in main-func.h into functions that we implement in main-func.c to
allow moving some included headers from the header to the .c file.

src/shared/meson.build

@@ -119,6 +119,7 @@ shared_sources = files(
         'machine-id-setup.c',
         'machine-pool.c',
         'macvlan-util.c',
+        'main-func.c',
         'mkdir-label.c',
         'mkfs-util.c',
         'module-util.c',