bgpd RFC compliance WRT received UPDATE attributes

A series of cases initiated by Jerry Scharf. Every commit present in RE-testing-0.99 so far seems to do its job. Some of the commits are merged, but some are not yet. Bugs marked "OK" have been confirmed by Jerry to be completely fixed.

master  RE-stable-0.99 RE-testing-0.99 bugzilla   summary
******* ************** *************** ********   *******
0ea968d d0511bd        6f3b7a9         674 (OK)   bgpd: check LOCAL_PREF attribute flags
214bcaa e531d4a        77a20b8         676/1 (OK) bgpd: add missing "partial" flag checks
N/A     9ebf400        5f28299         676/2      bgpd: fix AS4_PATH attr flag check
2cfadf0 7d25f18        197fb41         677 (OK)   bgpd: check MULTI_EXIT_DISC attr flags
9eba2ad ca22cc4        cfa5847         678/1 (OK) bgpd: check ATOMIC_AGGREGATE attr flags
b4cd242 12da1a1        36490db         678/2 (OK) bgpd: check AGGREGATOR attr flags
a624cae 047d6a6        d8fcead         679        bgpd: improve attr length error handling
bc3443e 3eca6f0        4561037         680 (OK)   bgpd: improve NEXT_HOP attribute checks
2d42e68 19e7654        7aff507                    bgpd: ignore 4 bits of attribute flags byte
b84b62d 395ec7f        2febf32                    bgpd: improve attr flags checks
a5b228b 566941f        05a4936                    bgpd: fix regression in improved attr flag checks
bbb04bf 7ebd470        b822752                    bgpd: fix more regressions in attr flag checks
d595b56 5de1719        e2afcb5                    bgpd: check ORIGINATOR_ID attribute flags
0b83044 cadc4cf        99bd4a7                    bgpd: check CLUSTER_LIST attribute flags
565b828 efb2c33        c8d74cb                    bgpd: add flag checks for MP_(UN)REACH_NLRI
afcb767 26f5e78        575d6fc                    bgpd: rewrite attr flag error logging
2675518 0f572cd        ffe3293                    bgpd: fix 2 more cases of length error reporting