unified build scheme: add a "unified" template for VMS descrip.mms

As part of this, change util/mkdef.pl to stop adding libraries to
depend on in its output.  mkdef.pl should ONLY output a symbol
vector.

Because symbol names can't be longer than 31 characters, we use the
compiler to shorten those that are longer down to 23 characters plus
an 8 character CRC.  To make sure users of our header files will pick
up on that automatically, add the DEC C supported extra headers files
__decc_include_prologue.h and __decc_include_epilogue.h.

Furthermore, we add a config.com, so VMS people can configure just as
comfortably as any Unix folks, thusly:

    @config

Reviewed-by: Rich Salz <rsalz@openssl.org>