Add support for additional windspeed units

The AWS-X station supports sending windspeeds in m/s, km/h and knots.
Since the transmitted packet does not contain which unit it uses, the
receiver (awsxd) must be configured with the same unit.

This patch adds a new command-line argument (-m) to select the unit,
which must correspond to the same unit configured at the station.

awsxd will internally store all windspeeds as m/s. For knots and km/h
transmissions, awsxd will convert the values before storage. For this
reason, this patch also alters the database structure for the windspeed
fields to FLOAT. See the table_structure.sql diff for details. For
migrating the old database content, you will want to divide all windspeed
fields (wind_avg, wind_max, wind_min and wind_daily_max) with 3.6 to
convert them from km/h to m/s.
Merge branch 'topic/speedunits'