[snmp/manager] Improve handling of unexpected/invalid snmpm_user callbacks

Improved handling of unexpected return values from snmpm_user
callback functions. Violations of the documented API (crashes or
invalid return values) will now result in an error message.
Updated doc for snmpm_user bahaviour and some other stuff.
Add more (common) type defs.
Update manager example.
Fixed manager test(s).
Fixed unused vars in (manager) test suite.
Make test user follow defined behaviour.
[snmp/agent] enable SNMP to create missing database directories

Add {db_init_error, create_db_and_dir} option to SNMP manager and
agent. This allows them to create any missing parent directories for
db_dir, rather than treating any missing directories as a fatal error.

The default for db_init_error, which is terminate, is unchanged.

Add create_db_and_dir to the documentation.

Add new tests to verify that using create_db_and_dir results in missing
parent directories being created.
[snmp] Update the config utility function for db-init-error

Update the config utility function (a simple cli for generating
the snmp config files) to handle the new db_init_error value
(create_db_and_dir).

OTP-11352
[snmp] Add proper version, release nodes and appup

Also fixed copyright end date.

OTP-11352
Merge branch 'bmk/snmp/manager/improve_user_callback_response_handling2/OTP-11307' into bmk/snmp/snmp425_integration

Conflicts:
	lib/snmp/doc/src/notes.xml
	lib/snmp/src/app/snmp.appup.src
{snmp] Fixed include depend
[snmp] Add (atl) log conversion block option

It is now possible to request that the Audit Trail Log should
be blocked during conversion (log_to_txt or log_to_io). This
could be usefull when coverting a large log (when there is
a chance it may otherwise wrap during conversion).
[snmp] Updated release notes, ref man, appup and assigned proper version
Merge branch 'bmk/snmp/blocking_log_during_conversion' into bmk/snmp/snmp425_integration

Conflicts:
	lib/snmp/doc/src/notes.xml
	lib/snmp/src/app/snmp.erl
[snmp] Commented out (currently) unused upgrade/downgrade function
[snmp] Wrong block cypher type used for AES

Wrong block cypher type used for AES ('aes_cbf128' instead of
'aes_cfb128') when performing AES block encrypt/decrypt which
breaks SNMP usmAesCfb128Protocol in agent and manager.

OTP-11412
[snmp/manager] Incorrect use of EngineBoots and EngineTime when encrypting

When performing the AES encryption, invalid values for the
EngineBoots and EngineTime was used. The values of the local
agent was used, which would have produced some values if an
agent was actually running. If not it would have caused a crash.

OTP-11413
Merge branch 'bmk/snmp/aes_encryption_fixes/OTP-11412' into bmk/snmp/snmp425_integration

Conflicts:
	lib/snmp/doc/src/notes.xml
[snmp] Improving ATL handling of corrupt logs

When converting an Audit Trail Log to text a corrupt
log entry could cause the entire conversion to fail.
Also, for a log with sequence numbers, failing to
decode a log entry would cause the conversion to fail
(not because of the failed decode, but because of the
failure to write the error message).

OTP-11453
Merge branch 'bmk/snmp/handling_corrupt_atl_when_converting/OTP-11453' into bmk/snmp/handling_corrupt_atl_when_converting/r16/OTP-11453

Conflicts:
	lib/snmp/doc/src/notes.xml
	lib/snmp/src/app/snmp.appup.src
	lib/snmp/vsn.mk
Merge branch 'bmk/snmp/handling_corrupt_atl_when_converting/r16/OTP-11453' into bmk/snmp/snmp425_integration

Conflicts:
	lib/snmp/src/misc/snmp_log.erl
[snmp] Loosing log entries when converting a large Audit Trail Log

When converting an entire Audit Trail Log in a running
system its possble to have log wraps and thereby loosing
log entries.
In order to prevent this the log is now blocked during
conversion (log_to_txt or log_to_io).
Merge branch 'bmk/snmp/handling_corrupt_atl_when_converting/OTP-11453' into bmk/snmp/snmp4223_integration

Conflicts:
	lib/snmp/doc/src/notes.xml
	lib/snmp/src/app/snmp.appup.src
	lib/snmp/src/misc/snmp_log.erl
[snmp] Cleanup and changed atl conversion block default

Changed the default value for the Audit Trail Log
conversion to true. Also some cleanup.

OTP-11396
Merge branch 'bmk/snmp/blocking_log_during_conversion' into bmk/snmp/snmp425_integration

Conflicts:
	lib/snmp/src/app/snmp.erl