Hurd: Adjust to startup-with-shell changes.

Hurd: Adjust to startup-with-shell changes.

In commit 98882a26513e25b2161b41dfd4bed97b59b2c01a, STARTUP_WITH_SHELL was made
a runtime toggle, startup-with-shell.  The Hurd code was missed to be adjusted;
it had a value hard-coded instead of using START_INFERIOR_TRAPS_EXPECTED.  Fix
that, and also simplify gnu-nat's pending_execs handling from counting to just
a flag.

	gdb/
	* gnu-nat.c (struct inf): Change pending_execs member to a 1-bit
	flag.  Adjust all users; in particular...
	(gnu_wait): ..., don't decrement its value in here...
	(gnu_create_inferior): ..., and instead set the flag in here,
	around the startup_inferior call, and call that one with
	START_INFERIOR_TRAPS_EXPECTED.