Break the thread of control that implies that a unix child

Break the thread of control that implies that a unix child
process will be the default target.

* target.c (find_default_run_target, find_default_attach,
find_default_create_inferior, return_zero):  new functions.
(cleanup_target):  Make return_zero the default for to_can_run.

* exec.c (exec_ops), core.c (core_ops):  Replace child_attach and
child_create_inferior references with find_default_XXX instead.

* target.h (struct target_ops): new field, to_can_run.
(find_default_attach, find_default_create_inferior):  new prototypes.
(target_can_run): new macro.

* Also added a zero (default) to_can_run element to all static
struct target_ops initializations throughout GDB, except:
* inftarg.c (child_ops): Use new child_can_run() to enable child runs.

* infrun.c (child_create_inferior):  Clean up error handling when
no exec file is specified.
(child_attach):  Don't require exec file.