Avoid potencially-stale errno usage.

Avoid potencially-stale errno usage.

The current throw_perror_with_name/TARGET_CLOSE_ERROR calls assume
errno is still set to the right error, although remote_unpush_target
is called in between, which may well change errno.

Tested on x86_64 Fedora 17 w/ gdbserver.

gdb/
2013-04-09  Pedro Alves  <palves@redhat.com>

	* remote.c (unpush_and_perror): New function.
	(readchar, remote_serial_write): Use it.