bsnmpd(8): fix and optimize interface description processing

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