forward-line now works with bignums

forward-line now works with bignums

* src/cmds.c (Fforward_line): Support bignum arg.
(scan_newline): Return void since no caller was using the
return value.
* src/search.c (find_newline, scan_newline_from_point)
(find_newline1): Return the number of newlines counted, not
the count shortage, so that the return value always fits in
ptrdiff_t even if the original count was a bignum.  All
callers changed.
* test/src/cmds-tests.el (forward-line-with-bignum): New test.