-Wwrite-strings: gdbserver's 'port' parsing

-Wwrite-strings: gdbserver's 'port' parsing

-Wwrite-strings flags this assignment as requiring a cast:

		  port = STDIO_CONNECTION_NAME;

because 'port' is a "char *", and STDIO_CONNECTION_NAME is a string
literal.

gdb/gdbserver/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

        * remote-utils.c (remote_prepare, remote_open): Constify.
        * remote-utils.h (remote_prepare, remote_open): Constify.
        * server.c (captured_main): Constify 'port' handling.