Fix VMS implicit rules and UNIX paths.

Fix VMS implicit rules and UNIX paths.

This fixes VMS implicit rules and UNIX style pathname handling.
It also fixes some of the VMS style pathname handling, more work
there will be needed later.
TODO: There are other case insensitive platforms besides VMS.
We need to find out why there is extra VMS code for this.  This
indicates either the extra VMS code is not needed, or the case
insensitive support may not be complete on the other case
insensitive platforms.

* default.c: Add missing definitions to default_suffix_rules[] and
default_variables[].
TODO: As it is important that VMS DCL mode definitions must always
be a superset of UNIX definitions, a better way of maintaining the
VMS DCL mode definitions should be devised.
* dir.c (downcase_inplace): Add a reentrant downcase() routine.
Add future support for VMS 8.2+ _USE_STD_STAT macro which will
disable a lot of VMS specific code from compiling.
(dir_file_exists_p): vmsify filename only if directory name has VMS
directory delimiters.
(file_exists_p): Handle both VMS and UNIX directories.
(file_impossible): Handle both VMS and Unix directories.  Track
whether a VMS format path is needed for the return value.
* file.c (lookup_file): Check if vmsify is needed; handle UNIX paths.
* implicit.c (pattern_search): Enable UNIX paths.
* read.c (parse_file_seq): Enable UNIX paths.
* remake.c (f_mtime): Fix gpath_search call for VMS paths.
* rule.c (count_implicit_rule): Enable UNIX paths, Fix VMS paths.
* vpath.c (selective_vpath_search): Enable UNIX paths.