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.
