os-posix: avoid buffer overrun

os_set_proc_name: Use pstrcpy, in place of strncpy and the
ineffectual preceding assignment: name[sizeof(name) - 1] = 0;

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