Add to bsnmpd(1) ability to specify multiple community strings with
different access rights.

By default there are two community strings with index 1 and 2, one for
read-only access and second for read-write access:

begemotSnmpdCommunityString.0.1 = $(read)
begemotSnmpdCommunityString.0.2 = $(write)

Now it is possible to define additional community strings using different
indexes:

begemotSnmpdCommunityString.0.3 = "SomeString1"
begemotSnmpdCommunityPermission.0.3 = 1
begemotSnmpdCommunityString.0.4 = "SomeString2"
begemotSnmpdCommunityPermission.0.4 = 2
begemotSnmpdCommunityString.0.5 = "SomeString3"
begemotSnmpdCommunityString.0.6 = "SomeString4"

New attribute begemotSnmpdCommunityPermission can be used to specify access
rights: 1 means "read-only" access, 2 means "read-write" access. If
attribute is not specified for some index this means "read-only" rights.

Community strings must be unique, i.e. must not be the same for different
indexes.

Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D13785