s3-lib Remove the clobber_region() code.

s3-lib Remove the clobber_region() code.

This code wrote to the full buffer in fstrcpy(), pstrcpy() and other
fixed-length string manipulation functions.

The hope of this code was to find out at run time if we were mixing up
pstring and fstring etc, and to record where this came from.  It has a
runtime performance impact (particularly if compiled with
--enable-developer).

It is being removed because of the complexity it adds, and the
distinct lack of bugs that this complexity has been credited in
finding.

The macro-based compile-time checking of string sizes remains.

Andrew Bartlett