Make the /proc support a target-struct in its own right.

Make the /proc support a target-struct in its own right.

* inferior.h (proc_wait, inferior_proc_init, proc_set_exec_trap):
No longer global functions.
(fork_inferior):  New global function from fork-child.c.
* inftarg.c (child_wait):  Remove USE_PROC_FS conditional.
(ptrace_me, ptrace_him):  New stub functions for fork_inferior().
(child_create_inferior):  Moved to fork-child.c as fork_inferior.
(child_create_inferior):  New tiny function that calls fork_inferior.
* fork-child.c:  New file, containing fork_inferior, which is
built from slight mods to inftarg.c's child_create_inferior.

* procfs.c (procfs_ops):  Add target vector.
(attach):  Rename as static do_attach.
(procfs_create_inferior):  New tiny function that calls fork_inferior.
(child_xfer_memory):  Rename to static procfs_xfer_memory.
(store_inferior_registers):  Rename to static procfs_store_registers.
(inferior_proc_init):  Rename to static procfs_init_inferior.
(procfs_attach, procfs_detach, procfs_prepare_to_store,
procfs_files_info, procfs_open, procfs_mourn_inferior,
procfs_can_run):  Slightly mangled copies of the corresponding
child_XXX routines from inftarg.c.
(proc_wait):  Renamed to static procfs_wait.
(child_resume):  Rename to static procfs_resume.
(fetch_inferior_registers):  Rename to static procfs_fetch_registers.
(initialize_proc_fs):  Rename to initialize_procfs.  Set up
procfs_ops, too.
* xm-irix4.h, xm-sysv4.h (CREATE_INFERIOR_HOOK):  No longer needed.