lib: use snmp's large fd sets for agentx

lib: use snmp's large fd sets for agentx

The maximum number of file descriptors in an fd set is limited by
FD_SETSIZE. This limitation is important because the libc macros
FD_SET(), FD_CLR() and FD_ISSET() will invoke a sigabort if the size of
the fd set given to them is above FD_SETSIZE.

We ran into such a sigabort with bgpd because snmp can return an fd set
of size higher than FD_SETSIZE when calling snmp_select_info(). An
unfortunate FD_ISSET() call later causes the following abort:

Received signal 6 at 1701115534 (si_addr 0xb94, PC 0x7ff289a16a7c); aborting...
/lib/x86_64-linux-gnu/libfrr.so.0(zlog_backtrace_sigsafe+0xb3) [0x7ff289d62bba]
/lib/x86_64-linux-gnu/libfrr.so.0(zlog_signal+0x1b4) [0x7ff289d62a1f]
/lib/x86_64-linux-gnu/libfrr.so.0(+0x102860) [0x7ff289da4860]
/lib/x86_64-linux-gnu/libc.so.6(+0x42520) [0x7ff2899c2520]
/lib/x86_64-linux-gnu/libc.so.6(pthread_kill+0x12c) [0x7ff289a16a7c]
/lib/x86_64-linux-gnu/libc.so.6(raise+0x16) [0x7ff2899c2476]
/lib/x86_64-linux-gnu/libc.so.6(abort+0xd3) [0x7ff2899a87f3]
/lib/x86_64-linux-gnu/libc.so.6(+0x896f6) [0x7ff289a096f6]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x2a) [0x7ff289ab676a]
/lib/x86_64-linux-gnu/libc.so.6(+0x1350c6) [0x7ff289ab50c6]
/lib/x86_64-linux-gnu/libc.so.6(+0x1366ab) [0x7ff289ab66ab]
/lib/x86_64-linux-gnu/libfrrsnmp.so.0(+0x36f5) [0x7ff2897736f5]
/lib/x86_64-linux-gnu/libfrrsnmp.so.0(+0x3c27) [0x7ff289773c27]
/lib/x86_64-linux-gnu/libfrr.so.0(thread_call+0x1c2) [0x7ff289dbe105]
/lib/x86_64-linux-gnu/libfrr.so.0(frr_run+0x257) [0x7ff289d56e69]
/usr/bin/bgpd(main+0x4f4) [0x560965c40488]
/lib/x86_64-linux-gnu/libc.so.6(+0x29d90) [0x7ff2899a9d90]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7ff2899a9e40]
/usr/bin/bgpd(_start+0x25) [0x560965c3e965]
in thread agentx_timeout scheduled from /build/make-pkg/output/_packages/cp-routing/src/lib/agentx.c:122 agentx_events_update()

Also, the following error is logged by snmp just before the abort:
snmp[err]: Use snmp_sess_select_info2() for processing large file descriptors

snmp uses a custom struct netsnmp_large_fd_set to work above the limit
imposed by FD_SETSIZE. It is noteworthy that, when calling
snmp_select_info() instead of snmp_select_info2(), snmp uses the same
code working with its custom, large structs, and copy/paste the result
to a regular, libc compatible fd_set. So there should be no downside
working with snmp_select_info2() instead of snmp_select_info().

Replace every use of the libc file descriptors sets by snmp's extended
file descriptors sets in agentx to acommodate for the high number of
file descriptors that can come out of snmp. This should prevent the
abort seen above.

Signed-off-by: Edwin Brossette <edwin.brossette@6wind.com>