analyzer: fixes to file-descriptor handling

gcc/analyzer/ChangeLog:
	* sm-fd.cc (fd_state_machine::on_open): Transition to "unchecked"
	when the mode is symbolic, rather than just on integer constants.
	(fd_state_machine::check_for_open_fd): Don't complain about
	unchecked values in the start state.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/fd-3.c (test_5): Expect "opened here" message
	even when flags are symbolic.
	(test_read_from_symbolic_fd): New.
	(test_write_to_symbolic_fd): New.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>