Simul efun: valid_mailaddr - check if a string could be an email address

Synopsis:
	mixed valid_mailaddr(string mailaddr, int silent)

Description:
	It checks if an email address MIGHT be a valid email address by having
	a closer look at the string: There has to be an '@' in it, it scans
	for not allowed characters in it, the part behind the '@' has to be
	a domain, etc. The second argument is optional und indicates to be
	verbose (0 = default) or silent.
	If the given string mailaddr MIGHT be an email address, the mailaddr
	is returned, else 0 is returned.

Return values:
	returns string mailaddr if it might be a valid email address
	returns 0 if it is not a valid email address
	
Example:
	mailaddr = valid_mailaddr(mailaddr);

Note:
	This is a simple validation of the format of the mail address. There
	are no checks via DNS if the domain exists etc.
	Email addresses @nemesis.de are not accepted.

See also: lib/valid_name