Add fallback to procfs after try netlink

[yuuki]

It gets the following error on an old CentOS5 (5.2).

[y_uuki@hoge ~]$ /home3/y_uuki/lstf
failed to get host flows: connection refused

the strace result.

socket(PF_NETLINK, SOCK_RAW, 4)         = 3
sendto(3, "L\0\0\0\22\0\1\3\0\0\0\0\0\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 76, 0, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = -1 ECONNREFUSED (Connection refused)

The ss command falls back to /proc/net/tcp.

socket(PF_NETLINK, SOCK_RAW, 4)         = 3
sendmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"L\0\0\0\22\0\1\3@\342\1\0\0\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 76}], msg_controllen=0, msg_flags=0}, 0) = -1 ECONNREFUSED (Connection refused)
open("/proc/net/tcp", O_RDONLY)         = 4

[coveralls]

Pull Request Test Coverage Report for Build 47

• 6 of 54 (11.11%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-5.7%) to 44.909%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
netutil/netutil_linux.go	1	2	50.0%
tcpflow/tcpflow_linux.go	5	8	62.5%
netutil/netutil.go	0	15	0.0%
tcpflow/tcpflow.go	0	29	0.0%

Totals
Change from base Build 45:	-5.7%
Covered Lines:	172
Relevant Lines:	383

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💛 - Coveralls