Fix a regression introduced by the at_keyboard.mod split. Because
some terminals are default on some platforms and non-default on
others, the first terminal being registered determines which is
going to be default.
* kern/term.c (grub_term_register_input): If this is the first
terminal being registered, set it as the current one.
(grub_term_register_output): Likewise.
* term/efi/console.c (grub_console_init): Do not call
grub_term_set_current_output() or grub_term_set_current_input().
* term/ieee1275/ofconsole.c (grub_console_init): Likewise.
* term/i386/pc/console.c (grub_console_init): Likewise.
(grub_console_fini): Do not call grub_term_set_current_input()
(but leave grub_term_set_current_output() to restore text mode).
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