Add xmemdupz() that duplicates a block of memory, and NUL terminates it.

Add xmemdupz() that duplicates a block of memory, and NUL terminates it.

A lot of places in git's code use code like:

  char *res;

  len = ... find length of an interesting segment in src ...;
  res = xmalloc(len + 1);
  memcpy(res, src, len);
  res[len] = '\0';
  return res;

A new function xmemdupz() captures the allocation, copy and NUL
termination.  Existing xstrndup() is reimplemented in terms of
this new function.

Signed-off-by: Pierre Habouzit <redacted>
Signed-off-by: Junio C Hamano <redacted>