Separate mouse-1-click-follows-link from mouse-drag-region.

Separate mouse-1-click-follows-link from mouse-drag-region.
* lisp/mouse.el (mouse--down-1-maybe-follows-link): New function.
(key-translation-map): Use it to implement mouse-1-click-follows-link.
(mouse-drag-line, mouse-drag-track): Remove mouse-1-click-follows-link code.
(mouse--remap-link-click-p): Remove.
* src/keyboard.c (access_keymap_keyremap): Accept nil return value from
functions to mean "no change".
* src/keyboard.h (EVENT_START, EVENT_END, POSN_WINDOW, POSN_POSN)
(POSN_WINDOW_POSN, POSN_TIMESTAMP): Be careful since events may come
from Elisp via unread-command-events.