This option wasn't converted to a proper data type when it was renamed
from "week_begins_on_monday" to "general.firstdayofweek". Convert the
boolean option into an enumeration type that can take the values
"monday" and "sunday". Also, update the documentation, add a conversion
rule to the upgrade script and convert the configuration file used in
the test suite.
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>