Hook scanning and indentation functions to find_newline. This helps

Hook scanning and indentation functions to find_newline.  This helps
to avoid duplicated code and renders more respect to newline cache.
* lisp.h (scan_newline): Prefer ptrdiff_t to EMACS_INT.
* cmds.c (Fforward_line):
* indent.c (scan_for_column, Fcurrent_indentation, indented_beyond_p):
Use find_newline and avoid unnecessary point movements.
* search.c (scan_newline): Implement on top of find_newline.