Phonet: provide pipe socket option to retrieve the pipe identifier

User-space sometimes needs this information. In particular, the GPRS
context or the AT commands pipe setups may use the pipe handle as a
reference.

This removes the settable pipe handle with CONFIG_PHONET_PIPECTRLR.
It did not handle error cases correctly. Furthermore, the kernel
*could* implement a smart scheme for allocating handles (if ever
needed), but userspace really cannot.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>