bt: replace fragile snprintf use and unwarranted strncpy

In bt_hci_name_req a failed snprintf could return len larger than
sizeof(params.name), which means the following memset call would
have a "length" value of (size_t)-1, -2, etc...  Sounds scary.
But currently, one can deduce that there is no problem:
strlen(slave->lmp_name) is guaranteed to be smaller than
CHANGE_LOCAL_NAME_CP_SIZE, which is the same as sizeof(params.name),
so this cannot happen.  Regardless, there is no justification for
using snprintf+memset.  Use pstrcpy instead.

Also, in bt_hci_event_complete_read_local_name, use pstrcpy in place
of unwarranted strncpy.

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