move select() from <unistd.h> to <sys/select.h>

this item is split off from bug 3172 which it was only related to in that the
header <sys/select.h> was receiving changes.

select is "defined" upstream (glibc, POSIX) as being in <sys/select.h>, and its
relative pselect is already in that header in LSB.

Commentary from bug 3172:
===
It's not easy to move interfaces between headers and preserve history, I think
the issue is not serious enough to try to implement such approach.

But we can try to simply move interface declaration to another header (update
Iheader field in the db) and see if it breaks anything. And then produce
corresponding errata, since this will affect all spec versions.
===
yes, I don't think ever in the past when we've moved interfaces we've worried
about history.
===
Then we can try this update:

UPDATE Interface SET Iheader=113 WHERE Iname='select';

Unsure whether this change needs an errata generated, have put 4.1-errata on
the list while we think about that (I'm inclined just to ignore it as it's only
this small change to the .defs)