Fix AdoNetAppender using npgsql

[NicholasNoise]

Using npgsql on netfx and mono I've encountered this error on an applicaiton startup:

log4net:ERROR [CustomAdoNetAppender] ErrorCode: GenericFailure. Exception while writing to database
Npgsql.PostgresException (0x80004005): 42601: syntax error at or near ":"
   at Npgsql.NpgsqlConnector.DoReadMessage(Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications, Boolean isPrependedMessage)
   at Npgsql.NpgsqlConnector.ReadMessage(Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications)
   at Npgsql.NpgsqlConnector.ReadMessage(Boolean async, DataRowLoadingMode dataRowLoadingMode, Boolean readingNotifications)
   at Npgsql.NpgsqlConnector.ReadExpecting[T](Boolean async)
   at Npgsql.NpgsqlCommand.Prepare()
   at log4net.Appender.AdoNetAppender.SendBuffer(IDbTransaction dbTran, LoggingEvent[] events)
   at log4net.Appender.AdoNetAppender.SendBuffer(LoggingEvent[] events)

The error prevents to write only very first logging event, because Npgsql fails to prepare command with no parameters (at a moment). So it is not a problem in general, but disturbing.

I think my fix is not that good but does a trick.

An alternative is to disable dbCmd.Prepare(); if it fails once, but it is overkill, cuz there is no second fail.

					if (m_doCommandPrepare)
					{
						try
						{
							// prepare the command, which is significantly faster
							dbCmd.Prepare();
						}
						catch (Exception)
						{
							m_doCommandPrepare = false;
						}
					}
...
		private bool m_doCommandPrepare = true;

[NicholasNoise]

@fluffynuts
At a look please.