Add binary relocation support for Windows platforms.

Add binary relocation support for Windows platforms.

* src/include/Makefile.sub (defs.h): Add `INSTALLPATH'.

* src/include/relocate.h, src/libs/libgroff/relocate.cpp: New files.

* src/libs/libgroff/searchpath.cpp: Handle `relocate'.
(search_path::search_path, search_path::open_file_cautious): Use
`relocate'.

* src/libs/libgroff/Makefile.sub (OBJS, CCSRCS),
src/include/Makefile.sub (HDRS): Updated.

* src/devices/grohtml/post-html.cpp: Don't use strdup/free but
strsave/a_delete.

* src/libs/libgroff/font.cpp: Replace strdup with strsave.

* src/libs/libgroff/maxfilename.cpp: Don't include limits.h since
lib.h already does it.

* src/libs/libgroff/maxpathname.cpp: New file, defining
path_name_max.

* src/include/lib.h: Add prototype for path_name_max.

* src/libs/libgroff/Makefile.sub (OBJS, CCSRCS): Updated.

* src/include/nonposix.h (PATH_EXT): New macro.