Revert 'Remove unused struct serial::name field'

This commit reverts:

 commit 5f5219fc34f7557296272230123a3837960a6f09
 Author:     Pedro Alves <palves@redhat.com>
 AuthorDate: Tue Apr 12 16:49:30 2016 +0100

     Remove unused struct serial::name field

The following patches will add uses for the field.

gdb/ChangeLog:
2020-01-10  Pedro Alves  <palves@redhat.com>

	Revert:
	2016-04-12  Pedro Alves  <palves@redhat.com>
	* serial.c (serial_open, serial_fdopen_ops, do_serial_close):
	Remove references to name.
	* serial.h (struct serial) <name>: Delete.