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 altoget

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

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