[BZ #16046] Static dlopen correction fallout fixes.

Fixes to address issues from BZ #15022 resolution, as follows:

* TLS updates to csu/libc-tls.c -- we now have a proper main map, so
  there's no longer a need to create a separate fake one to keep TLS
  structures,

* random updates to elf/dl-close.c -- LM_ID_BASE is now a valid name
  space ID for static executables as well, so assert that we don't
  unload the main map.  Similarly dl_nns isn't supposed to be 0 for
  static executables anymore,

* actual BZ #16046 fix to elf/dl-iteratephdr.c -- the dl_iterate_phdr
  special function for static executables isn't needed anymore, provided
  that l_phdr and l_phnum members of the main map have been properly
  initialized (done in _dl_non_dynamic_init in elf/dl-support.c now),

* ld.so.cache loader update to elf/dl-load.c --
  GL(dl_ns)[LM_ID_BASE]._ns_loaded is now always initialized in static
  executables so can become the fallback loader map to check for
  DF_1_NODEFLIB, provided that the l_flags_1 member of the main map has
  been properly initialized (done in elf/dl-support.c now); this also
  ensures previous semantics elsewhere in elf/dl-load.c,

* matching updates to elf/dl-support.c -- to complement the two fixes
  above.
* manual/ctype.texi: Document MTASC-safety properties.
* manual/getopt.texi: Document MTASC-safety properties.
* manual/job.texi: Document MTASC-safety properties.
* manual/lang.texi: Document MTASC-safety properties.
* manual/libdl.texi: New.
* manual/llio.texi: Document MTASC-safety properties.
* manual/locale.texi: Document MTASC-safety properties.
* manual/math.texi: Document MTASC-safety properties.
[BZ #12751]
* manual/memory.texi: Document MTASC-safety properties.
* manual/message.texi: Document MTASC-safety properties.
* manual/pattern.texi: Document MTASC-safety properties.
* manual/pipe.texi: Document MTASC-safety properties.
* manual/platform.texi: Document MTASC-safety properties.
* manual/process.texi: Document MTASC-safety properties.
* manual/resource.texi: Document MTASC-safety properties.
* manual/search.texi: Document MTASC-safety properties.
* manual/setjmp.texi: Document MTASC-safety properties.
* manual/signal.texi: Document MTASC-safety properties.
* manual/socket.texi: Document MTASC-safety properties.