bgpd: Use strict AS4 capability when processing parsing/generating pkts

bgpd: Use strict AS4 capability when processing parsing/generating pkts

PeerA sets `dont-capability-negotiate` for PeerB. It does not send any
capabilities to PeerB. This leads to situation when PeerA received AS4 cap,
while it doesn't send AS4 to PeerB and tries parsing AS_PATH using 32bits.

[GTTPK-RX2GP][EC 33554436] Malformed AS path from 192.168.0.2, length is 4
[SM0KX-WXMGK] bgp_attr_malformed: attributes: , origin ?
[P7TRR-4J6XT][EC 33554487] 192.168.0.2: Attribute AS_PATH, parse error - treating as withdrawal
[WX70K-6XXVF][EC 33554454] 192.168.0.2 rcvd UPDATE with errors in attr(s)!! Withdrawing route.
[Z2KFR-ZGJC8] 192.168.0.2 rcvd UPDATE w/ attr: , origin ?
[P9VY3-5B6NX] 192.168.0.2 rcvd UPDATE wlen 0 attrlen 80 alen 73
[QMZ79-K2DH7][EC 33554454] 192.168.0.2 [Error] Update packet error (wrong prefix length 64 for afi 1)
[P9SYB-54XRZ][EC 33554454] 192.168.0.2 [Error] Error parsing NLRI
[V1CHF-JSGRR] %NOTIFICATION: sent to neighbor 192.168.0.2 3/10 (UPDATE Message Error/Invalid Network Field) 0 bytes
[HTHRX-GQYGJ][EC 33554454] bgp_process_packet: BGP UPDATE receipt failed for peer: 192.168.0.2

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>