PR remote/21188: Fix remote serial timeout

PR remote/21188: Fix remote serial timeout

As Gareth McMullin <gareth@blacksphere.co.nz> reports at
<https://sourceware.org/ml/gdb-patches/2017-02/msg00560.html>, the
timeout mechanism in ser-unix.c was broken by commit 048094acc
("target remote: Don't rely on immediate_quit (introduce quit
handlers)").

Instead of applying a local fix, and since we now finally always use
interrupt_select [1], let's get rid of hardwire_readchar entirely, and
use ser_base_readchar instead, which has similar timeout handling,
except for the bug.

Smoke tested with:

 $ socat -d -d pty,raw,echo=0 pty,raw,echo=0
 2017/03/14 14:08:13 socat[4994] N PTY is /dev/pts/14
 2017/03/14 14:08:13 socat[4994] N PTY is /dev/pts/15
 2017/03/14 14:08:13 socat[4994] N starting data transfer loop with FDs [3,3] and [5,5]
 $ gdbserver /dev/pts/14 PROG
 $ gdb PROG -ex "tar rem /dev/pts/15"

and then a few continues/ctrl-c's, plus killing gdbserver and socat.

[1] - See FIXME comments being removed.

gdb/ChangeLog:
2017-03-17  Pedro Alves  <palves@redhat.com>

	PR remote/21188
	* ser-base.c (ser_base_wait_for): Add comment.
	(do_ser_base_readchar): Improve comment based on the ser-unix.c's
	version.
	* ser-unix.c (hardwire_raw): Remove reference to
	scb->current_timeout.
	(wait_for, do_hardwire_readchar, hardwire_readchar): Delete.
	(hardwire_ops): Install ser_base_readchar instead of
	hardwire_readchar.
	* serial.h (struct serial) <current_timeout, timeout_remaining>:
	Remove fields.