lib: changes for making snmp socket non-blocking

lib: changes for making snmp socket non-blocking

Description: The changes have been done to make the snmp socket
non-blocking before calling snmp_read()

Problem Description/Summary :
vtysh hangs on first try to enter after a reboot with BGP dynamic peers

Expected Behavior :
VTYSH should not hang.

When we debug more into bgpd docker by doing gdb on its threads, we find the below thread of bgpd, which is causing the issue.
Thread 1 (Thread 0x7f1e1ec46d40 (LWP 47)):

0x00007f1e1d762593 in recvfrom () from /lib/x86_64-linux-gnu/libpthread.so.0
0x00007f1e1aadd09b in netsnmp_tcpbase_recv () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1aad9617 in netsnmp_transport_recv () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1aab2c07 in _sess_read () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1aab3a29 in snmp_sess_read2 () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1aab3a7b in snmp_read2 () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1aab3acf in snmp_read () from /usr/lib/x86_64-linux-gnu/libnetsnmp.so.30
0x00007f1e1b44d7ec in agentx_read (t=0x7fffa75f0080) at lib/agentx.c:63
0x00007f1e1e7d6451 in thread_call (thread=0x7fffa75f0080) at lib/thread.c:1620
0x00007f1e1e770699 in frr_run (master=0x559396ea60f0) at lib/libfrr.c:1011
0x0000559395b4d953 in main (argc=5, argv=0x7fffa75f02b8) at bgpd/bgp_main.c:492

(gdb) bt

0x00007f830c89d210 in __read_nocancel () from /lib/x86_64-linux-gnu/libpthread.so.0
0x000056450e1e8238 in vtysh_client_run (vclient=0x56450e4a8b40 <vtysh_client+24768>, line=0x56450e21add0 enable, callback=0x0, cbarg=0x0) at vtysh/vtysh.c:216
0x000056450e1e8c6b in vtysh_client_run_all (head_client=0x56450e4a8b40 <vtysh_client+24768>, line=0x56450e21add0 enable, continue_on_err=0, callback=0x0, cbarg=0x0) at vtysh/vtysh.c:356
0x000056450e1e8ddb in vtysh_client_execute (head_client=0x56450e4a8b40 <vtysh_client+24768>, line=0x56450e21add0 enable) at vtysh/vtysh.c:393
0x000056450e1e9c82 in vtysh_execute_func (line=0x56450e21add0 enable, pager=0) at vtysh/vtysh.c:598
0x000056450e1e9dee in vtysh_execute_no_pager (line=0x56450e21add0 enable) at vtysh/vtysh.c:619
0x000056450e1f7d48 in vtysh_read_file (confp=0x56451000a9d0, top_cfg=1) at vtysh/vtysh_config.c:494
0x000056450e1f7ef2 in vtysh_read_config (config_default_dir=0x56450e4edc20 <frr_config> /etc/frr/frr.conf, top_cfg=1) at vtysh/vtysh_config.c:522
0x000056450e1e5de4 in vtysh_apply_top_level_config () at vtysh/vtysh_main.c:301
0x000056450e1e7842 in main (argc=2, argv=0x7ffc81e6f598, env=0x7ffc81e6f5b0) at vtysh/vtysh_main.c:692

The fix has been taken from the following link.
https://sourceforge.net/p/net-snmp/patches/1348/

Signed-off-by: Preetham Singh <preetham.singh@broadcom.com>