grep: change some int types.

Change several int variables to size_t, ssize_t, or ptrdiff_t.
This should fix the bug described in CVE-2012-5667 when an input
line is so long that its length cannot be stored in an int
variable.

This is based on NetBSD's revision which says:

This change to NetBSD's version of GNU grep 2.5.1 (licenced under
GPLv2) was made without direct reference to any code licenced
under GPLv3.

Obtained from:	NetBSD
MFC after:	3 days