Make STARTUP_WITH_SHELL a runtime toggle -- add new "set/show startup-with-shell...

Make STARTUP_WITH_SHELL a runtime toggle -- add new "set/show startup-with-shell" option.

Occasionaly we hear about people having problems with GDB not being
able to start programs (with "run"/"start").  GDB spawns a shell to
start the program, and most often, it'll be the case that the problem
is actually with the user's shell setup.

GDB has code to disable the use of the shell to start programs.
That's the STARTUP_WITH_SHELL macro that native targets could set to 0
in their nm.h file (though no target actually uses it nowadays).

This patch makes that setting a run-time knob instead.  This will be
useful to quickly diagnose such shell issues, and might also come in
handy at other times (such as when debugging the shell itself, if you
don't have a different shell handy).

gdb/
2013-10-24  Pedro Alves  <palves@redhat.com>

	* NEWS (New options): Mention set/show startup-with-shell.
	* config/alpha/nm-osf3.h (START_INFERIOR_TRAPS_EXPECTED): Set to 2
	instead of 3.
	* fork-child.c (fork_inferior, startup_inferior): Handle 'set
	startup-with-shell'.
	(show_startup_with_shell): New function.
	(_initialize_fork_child): Register the set/show startup-with-shell
	commands.
	* inf-ptrace.c (inf_ptrace_create_inferior): Remove comment.
	* inf-ttrace.c (inf_ttrace_him): Remove comment.
	* procfs.c (procfs_init_inferior): Remove comment.
	* infcmd.c (startup_with_shell): New global.
	* inferior.h (startup_with_shell): Declare global.
	(STARTUP_WITH_SHELL): Delete.
	(START_INFERIOR_TRAPS_EXPECTED): Set to 1 by default instead of 2.

gdb/doc/
2013-10-24  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Starting): Document set/show startup-with-shell.