Check for interrupts in bf and xdes crypt()

Hi,

Here is a patch which makes it possible to cancel a query which runs the
crypt() function with the bf or xdes hashing algorithm, e.g.
crypt('foo', gen_salt('bf', 13)). The md5 algorithm does not run for
multiple rounds so there is no reason to patch it.

I noticed this problem when I accidentally picked a too high n for the
number of hash rounds.

I have added a call to CHECK_FOR_INTERRUPTS() after every round, and I
could not measure any performance hit from this.

Andreas