Remove some useless casts to (void *)

I have found that in some corners of the code some calls to standard C 
functions are decorated with casts to (void *) for no reason, and this 
code pattern then gets copied around.  I have gone through and cleaned 
this up a bit, in the attached patches.

The involved functions are: repalloc, memcpy, memset, memmove, memcmp, 
qsort, bsearch

Also hash_search(), for which there was a historical reason (the 
argument used to be char *), but not anymore.