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 green have been confirmed by Jerry to be completely fixed.

mapping of commits

bug ID	summary	RE-testing-0.99 commit	master commit
674	bgpd: check LOCAL_PREF attribute flags	6f3b7a900c89798a5fe4ae22f602180a820dbcc5	0ea968d21f194b3960a73aa47a5b06f160632907
676, pt.1	bgpd: add missing "partial" flag checks	77a20b8af32c926176f8492a086d9e4ec670496b	214bcaa13e092d9fff8f233e62ba28ca7eefbc43
676, pt.2	bgpd: fix AS4_PATH attr flag check	5f28299f8d615e0d4457c11160378f112272eab1	not required
677	bgpd: check MULTI_EXIT_DISC attr flags	197fb41fa4cdf418a69a9363ec6c0fea3e0cdd4d	2cfadf09a810e3b660a47a1eb898d0d7c8813c08
678 pt.1	bgpd: check ATOMIC_AGGREGATE attr flags	cfa5847748a645afe11a8b6c29cbd72cf77cd41d	9eba2ada189597fcb39c806bbbed414fecf20623
678 pt.2	bgpd: check AGGREGATOR attr flags	36490db664ba0f7a0ddfe32f5492c24594bc2d16	b4cd2421518be7e18046e974e25b9bf4cf769bd2
679	bgpd: improve attr length error handling	d8fceadb3ad03356fb8f620c8ac1991f3e719f71	a624cae2b210a0e81c80c473f86b73e2be169962
680	bgpd: improve NEXT_HOP attribute checks	4561037b2bb93aa1f546291f63883e39bd1fe93c	bc3443ebf032b5fcc9e0ccb94641e4e899cd17d8
	bgpd: ignore 4 bits of attribute flags byte	7aff5078ea538d13d8101b60b1c233462a7d6630	2d42e68aa032ed2f11471aee444935918d35c8bb
	bgpd: improve attr flags checks	2febf323411c1aed9d7694898f852ce2ef36a7e5	b84b62dfb6ee9daf46c9e4c2c372b179f33be44c
	bgpd: fix regression in improved attr flag checks	05a4936b713b9882171d0f7fb20b8439df23939e	a5b228b3792937c93b589938a2545b9311b7938e
	bgpd: fix more regressions in attr flag checks	b822752014ed7dfa918ca03780624854516fe77c	bbb04bf3bb03be27fbf17b6b3ee903b168201cf0
	bgpd: check ORIGINATOR_ID attribute flags	e2afcb525aae2f0a4d3de91f77f01dcf1a955d13	d595b566bca1de8b026c469a31ae50f2aee11781
	bgpd: check CLUSTER_LIST attribute flags	99bd4a72b878bf655bd382cc15d87f6f0cb118cf	0b83044b91d7ea50824cfd803d77213b9ff38ed9
	bgpd: add flag checks for MP_(UN)REACH_NLRI	c8d74cb62b753cfe3e20898c37db8c59e6949396	565b828dc00cafd477dd69ce15f0f551ece67710
	bgpd: rewrite attr flag error logging	575d6fc0ecf950955c44bc630d171b24ec2db52a	afcb767922509c4d998f1c567e350b9809c148ab
	bgpd: fix 2 more cases of length error reporting	ffe32937fce002f064e98e8cfdfacdda7670769f	267551879e57a5c590f9e12b3a7d6985b28b3dac