Fixes for pty handling in gdb-mi.el and process.c.

Fixes for pty handling in gdb-mi.el and process.c.

* lisp/progmodes/gdb-mi.el (gdb): Revert 2012-04-19 change.
(gdb-inferior-io--init-proc): New function.
(gdb-init-1): Use it.
(gdb-inferior-io-sentinel): New sentinel for the gdb-inferior pty,
responsible for allocating a new pty and hooking it to gdb when
the old pty gets an EIO due to process exit.
(gdb-delchar-or-quit): New command.  Bind it in gdb-mi buffers.
(gdb-tooltip-print): Don't use obsolete tooltip-use-echo-area.
(gdb-inferior-io--maybe-delete-pty): Move into gdb-reset.

* src/process.c (wait_reading_process_output): If EIO occurs on a pty,
set the status to "failed" and ensure that sentinel is run.

* doc/lispref/processes.texi (Asynchronous Processes): Mention nil
argument to start-process.