* Makefile.in: Add rule for gnu-nat.o and i386gnu-nat.o (Gnu Hurd)
* config/i386/i386gnu.mh: remove rules for [i386]gnu-nat.o, now
in Makefile.in (as for other targets); add NATDEPFILE corelow.o to
satisfy symbol dependancy in solib.c (core_ops).
* target.[ch] conditionalize Mach-specific signals so that they
won't show up in non-Mach gdb's!
* thread.c: add function thread_id_to_pid; change name of static
function "thread_switch" to "switch_to_thread", to avoid conflict
with Mach global symbol; move thread_cmd_list to global scope
so targets can add their own thread commands.
* infrun.c: sig_print_info: allow for long signal names.
* gnu-nat.[ch]: tidying up comments.
* gnu-nat.c: move several functions to static scope, just for tidy;
remove calls to prune_threads and renumber_threads;
gnu_wait must not return -1 when inferior exits;
attach_to_child will modify inferior_pid in a way that allows
fork_inferior to remain unchanged;
remove extra arg from startup_inferior;
move mach-specific thread commands here from thread.c.
