doc: update HACKING wrt strncpy/pstrcpy

Reword the section on strncpy: its NUL-filling is important
in some cases.  Mention that pstrcpy's signature is different.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
