Fix visibility of various functions in a number of headers.

Fix visibility of various functions in a number of headers.

The following functions were moved from XSI into the base with
POSIX 200809:

catclose(), catgets(), catopen(), dlclose(), dlerror(), dlopen(),
dlsym(), fchdir(), getpgid(), getsid(), getsubopt(), iconv(),
iconv_close(), iconv_open(), lchown(), mkstemp(), nl_langinfo(),
poll(), pread(), pwrite(), strdup(), strfmon(), tcgetsid(),
truncate(), and waitid().

This commit adjusts our headers to reflect that, i.e. specifying
_POSIX_C_SOURCE=200809 is now enough to expose their prototypes.

The checking of __XSI_VISIBLE's value (ranges) for XSI functions
could certainly be better (generally in our headers).

Also, we don't yet have waitid().

This change was build tested with around 8000 ports (until the
bulk build choked on a jdk7 memory leak (I suppose) while building
textproc/fop). The glibc conformance test suite thinks this
commit is correct, too.