Simul efun: valid_name - check if a string is a valid name for a player/wizard

Synopsis:
	status valid_name(string name)

Description:
	It tests if a string is a valid name for a player here: In lower
	case it has to be composed of the characters 'a' to 'z' and must
	have a length of at least 2 and max. 13 characters.

Return values:
	returns 1 if the name would be a valid name for a player
	returns 0 if it is not a valid name

Example:
	if (valid_name(name))
		write("This is a good name for an adventurer.\n");
	else write("No player ever would have this name.\n");

Note:
	Currently no check if name is banished.

Note:
	There are no checks if there is a player/wizard with this name.

See also: lib/user_exists, lib/valid_mailaddr, build/banish