Add to bsnmpd(1) ability to specify multiple community strings with

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