sysdeps/generic changes

[Roland McGrath]

I tried to do a somewhat exhaustive change of #include "" to <> in cases
where it might have been wrong.  I left many "" cases that were clearly
right or clearly didn't matter, though I also changed some that didn't
really matter.  Hopefully I changed all the ones that matter and didn't
change any that shouldn't have been changed.

After this it's probably possible to move header files from sysdeps/generic
into respective subdirs for headers that are only referenced within their
own subdir (the top-level subdir whose Makefile lists that header).
Similarly, nptl/sysdeps/{pthread,generic} files that are only used in the
libpthread build can move to nptl/.