Re: performance work (ITS#641)

[Kurt@OpenLDAP.org]

These changes were fairly easy to review.

SAFEMEMCPY suggested change is not safe (overlapping regions).
Per analysis of others, likely only a simple (and SAFE)
len==1 optimization would be yield performance
improvement across most platforms.

man alloca(3)
   BUGS: The alloca() function is machine dependent; its use is discouraged.

connection.c/tmpdn change applied.

fast_strs/fast_cstr optimizations
  may be worth generalizing and extended to support buffer overrun
  detection