* source.c (mod_path, openp): Use HAVE_DOS_BASED_FILE_SYSTEM

	* source.c (mod_path, openp): Use HAVE_DOS_BASED_FILE_SYSTEM
	instead of system-specific define's like _WIN32 and __MSDOS__.
	Use IS_DIR_SEPARATOR and IS_ABSOLUTE_PATH instead of SLASH_P and
	ROOTED_P.
	(top-level): #include "filenames.h".

	* solib.c (solib_open): Use IS_DIR_SEPARATOR and IS_ABSOLUTE_PATH
	instead of SLASH_CHAR, ROOTED_P and SLASH_P.
	(top-level): #include "filenames.h".

	* defs.h (SLASH_P, SLASH_CHAR, ROOTED_P): Remove definitions.
	(SLASH_STRING): Define only for _WIN32.

	* completer.c: Use HAVE_DOS_BASED_FILE_SYSTEM instead of
	__MSDOS_.

	* cli/cli-cmds.c (cd_command): Use IS_DIR_SEPARATOR and
	IS_ABSOLUTE_PATH instead of SLASH_P and ROOTED_P.  Replace
	system-specific ifdefs with HAVE_DOS_BASED_FILE_SYSTEM.
	(top-level): #include "filenames.h".