Configurations/descrip.mms.tmpl: Change strategy for include directories

Configurations/descrip.mms.tmpl: Change strategy for include directories

Instead of what we used to do, put all include directories in a number
of DCL variables and generate the /INCLUDE qualifier value on the
command line, we instead generate VMS C specific header files with
include directory pragmas, to be used with the VMS C's /FIRST_INCLUDE
qualifier.  This also shortens the command line, the size of which is
limited.

VMS C needs to have those include directories specified in a Unix
form, to be able to safely merge #include paths with them when
searching through them.

Fixes #14247

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15317)