Add MPI_T.5 man page for Open MPI-specific info

Also added infrastructure to have developers write man pages in
Markdown (vs. nroff).  Pandoc >=v1.12 is used to convert those
Markdown files into actual nroff man pages.

Dist tarballs will contain generated nroff man pages; we don't want to
require users to have Pandoc installed.  Anyone who builds Open MPI
from a git clone will need to have Pandoc installed (similar to how we
treat Flex).  You can opt out of Open MPI's Pandoc-generated man pages
by configuring Open MPI with --disable-man-pages.  This will also
disable "make dist" (i.e., "make dist" will error if you configured
with --disable-man-pages).

Also removed the stuff to re-generate man pages.

This commit also:

1. Includes a new man page, written in Markdown
   (ompi/mpi/man/man5/MPI_T.5.md) that contains Open MPI-specific
   information about MPI_T.
2. Includes a converted ompi/mpi/man/man3/MPI_T_init_thread.3.md (from
   MPI_T_init_thread.3in -- i.e., nroff) just to show that Markdown
   can be used throughout the Open MPI code base for man pages.
3. Made the Makefiles in ompi/mpi/man/man?/ be full-fledged
   Makefile.am's (vs. Makefile.extras that are designed to be included
   in ompi/Makefile.am).  It is more convenient to test generation /
   installation of man pages when you can "make" and "make install" in
   their respective directories (vs. doing a build / install for the
   entire ompi project).
4. Removed logic from ompi/Makefile.am that re-generated man pages if
   opal_config.h changes.

Other man pages -- hopefully all of them! -- will be converted to
Markdown over time.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>