[PATCH] Merge tinrc.*_goto_next_unread to tinrc.goto_next_unread

Hello,

the attached patch merges tinrc.{pgdn_goto_next,tab_goto_next_unread,
space_goto_next_unread} into one option tinrc.goto_next_unread (see
doc/TODO).
I've also merged the key PAGE_PAGE_DOWN3 (SPACE) into GLOBAL_PAGE_DOWN.
Everywhere PageDown and Space are mapped to GLOBAL_PAGE_DOWN except in
the page-level. To be consistent with the others levels SPACE is now
also GLOBAL_PAGE_DOWN. As a consequence tinrc.goto_next_unread
distinguishes now only between PageDown (PgDn and SPACE) and
PageNextUnread (TAB).
If this change is not wanted I will provide a patch with the first part
only.

Michael