Remove useless casts to (char *)

In the spirit of the recent patch set "Remove useless casts to (void *)"
[0], here is patch set that removes a bunch of apparently useless casts
to (char *).

There are two larger themes:

1) Various casts around string/memory functions such as strcpy() or
memcpy() that pretty much don't make any sense at all (at least
post-1989 I guess).

2) Using void * instead of char * for function arguments that deal with
binary data. The largest of these is XLogRegisterData() and
XLogRegisterBufData(), which were also mentioned in [0]. (similar past
patches: 2d4f1ba6cfc 1f605b82ba6 3b12e68a5c4 b9f0e54bc95)

The remaining (char *) casts are mostly related to signed/unsigned
conversion, controlling pointer arithmetic, and related to
palloc/malloc, (and probably some I missed or didn't want to touch) so
those are all ok to keep.

[0]:
https://www.postgresql.org/message-id/flat/461ea37c-8b58-43b4-9736-52884e862820%40eisentraut.org