Use enum flags for flags passed to openp

Use enum flags for flags passed to openp

gdb/ChangeLog:

	* defs.h (enum openp_flags): New enum.
	(OPF_TRY_CWD_FIRST, OPF_SEARCH_IN_PATH, OPF_RETURN_REALPATH):
	Move to enum openp_flags.
	(openp_flags): New enum flags.
	(openp): Change parameter type to openp_flags.
	* source.c (openp): Change parameter type to openp_flags.
	* cli/cli-cmds.c (find_and_open_script): Use openp_flags.
	* dwarf2read.c (try_open_dwop_file): Use openp_flags.