FreeBSD 15.0-ALPHA1+ exposes TCP pcblist entries as struct xtcpcb, where t_state
resides. Net-SNMP's tcpTable.c still treated entries as struct xinpcb, causing
compile errors (no member t_state / xt_inp path mismatch).
This change:
- Includes <sys/param.h>, <netinet/tcp_var.h>, <netinet/in_pcb.h> on FreeBSD.
- Sets NS_ELEM to struct xtcpcb on __FreeBSD_version >= 1500064.
- Keeps the old xinpcb path on older systems.
No functional change expected on 14.x; fixes build on 15.0-ALPHA1+.
PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=289351