[th/fix-ifcfg-infiniband-p-key] ifcfg: better handle non-full-membership PKEY_ID

Handling of p-keys with non-full-membership flag (0x8000) was broken with ifcfg-rh.

Earlier attempts to fix that (!1357 (merged), !1637 (merged)) were incomplete/wrong. With this change, everything (about this topic) works, while being backward compatible and supporting the cases that used to work previously.

See the commit message for details.