module: relocate module_init from init.h to module.h

Modular users will always be users of init functionality, but
users of init functionality are not necessarily always modules.

Hence any functionality like module_init and module_exit would
be more at home in the module.h file.  And module.h should
explicitly include init.h to make the dependency clear.

We've already done all the legwork needed to ensure that this
move does not cause any build regressions due to implicit
header file include assumptions about where module_init lives.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>