bsnmpd(8): fix and optimize interface description processing

• correctly prepare a buffer to obtain interface description from a kernel and truncate long description instead of dropping it altogether and spamming logs;
• skip calling strlen() for each description and each SNMP request for MIB-II/ifXTable's ifAlias.
• teach bsnmpd to allocate memory dynamically for interface descriptions to decrease memory usage for common case and not to break if long description occurs;

PR: 217763
Reviewed by: harti and others
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16459