infiniband-diags: add common -Y/--pkey-idx option

The diagnostic tools always sent their SA and GMP MADs with P_Key index 0
because the pkey_idx field of the destination ib_portid_t was never
populated.  On fabrics that place management traffic on a non-default
partition this prevents the tools from reaching the SA/PM.

Add a -Y/--pkey-idx option to the shared ibdiag option parser so every
tool that issues MADs can select the P_Key table index of its requests.
The value is validated as a uint16_t (0..65535, default 0; negative,
non-numeric and out-of-range values are rejected) and stored in the
ib_portid_t used to build each MAD, where mad_build_pkt() hands it to
umad_set_pkey() for non-SMP classes (SMPs always use index 0).

The index is also applied to an SM port id resolved during option
parsing, so it takes effect regardless of whether -Y/--pkey-idx precedes
or follows --sm_port.  The option is documented in the common and
per-tool man pages.

Signed-off-by: Jonathan Süssemilch Poulain <jpoulain@coreweave.com>