Re: Cleaning up historical portability baggage

Here's a new batch of these:

Remove configure probe for sys/uio.h.
Remove configure probes for sys/un.h and struct sockaddr_un.
Remove configure probe for sys/select.h.
Remove configure probes for sys/ipc.h, sys/sem.h, sys/shm.h.
Remove configure probe for sys/resource.h and refactor.
Remove configure probe for shl_load library.

The most interesting things to say about these ones are:
 * The concept of a no-Unix-socket build is removed.  We should be
able to do that now, right?  Peter E seemed to say approximately that
in the commit message for 797129e5.  Or is there a thought that a new
operating system might show up that doesn't have 'em and we'd wish
we'd kept this stuff well marked out?
 * Instead of eg HAVE_SYS_RESOURCE_H I supplied <sys/resource.h> and
moved the relevant declarations there from the big random-win2-stuff
header, and likewise for <sys/un.h>

(I still plan to get rid of no-threads-builds soon, but I got stuck
figuring out how to make sure I don't break the recent magic ldap
library detection... more soon.)