Use bool in signatures of functions in normal.h.

Return bool from checkclearop, checkclearopq, add_to_showcmd,
find_decl, nv_screengo, get_visual_text, and unadjust_for_sel as these
functions all return either TRUE or FALSE or OK or FAIL, but not MAYBE.

Change to arguments toplevel of normal_cmd, check of
do_check_scrollbind, and locally and thisblock of finddecl, to bools as
they represent predicates.