Not in working condition, things need to be moved around to work with
generation of AC code in a separate file.

This is required to be able to include the generated AC code from
clinic/selectmodule.c.h, which must appear before these definitions
but after all typedefs and type declarations.

The converter was copied from Modules/posixmodule.c, but changed to
check whether fd == -1 rather than fd < 0 as an error condition from
calling PyOjbect_AsFileDescriptor().

Note: AC output in Modules/clinic/selectmodule.c.h was manually
modified in this commit to get the build working. This may be due to
an AC bug.

Note: AC output in Modules/clinic/selectmodule.c.h was manually
modified in this commit to get the build working. This may be due to
an AC bug.

Also change the doc-string of epoll.poll:
* use AC's per-argument docs to describe the arguments
* revert an addition describing the return value

Note: AC output in Modules/clinic/selectmodule.c.h was manually
modified in this commit to get the build working. This may be due to
an AC bug.