add support for -fmemcpy-max-count

add support for -fmemcpy-max-count

By default, sparse will warn if memcpy() (or memset(),
copy_from_user(), copy_to_user()) is called with a very large
static byte-count.

But the limit is currently fixed at 100000, which may be fine
for some uses but not for others. For example, this value is
too low for sparse to be used on the git tree where, for example,
some array used to sort the index is cleared with memset().

Change this by making the limit configurable via a new flag:
-fmemcpy-max-count.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>