ftpfs change to compensate for some servers

[presotto@plan9.att.com presotto@plan9.att.com]

	some FTP servers include . and .. in the list, which messes up
	common commands applied to a mounted ftpfs directory.
	the following change prevents them from being seen:

	vortex% diff $home/cd/cmd/ftpfs /sys/src/cmd/ftpfs/proto.c
	38a39
	> static int	isdotdot(char*);
	670a672,673
	> 			if(isdotdot(field[7]))
	> 				return 0;
	681a685,686
	> 			if(isdotdot(field[8]))
	> 				return 0;
	692a698,699
	> 			if(isdotdot(field[9]))
	> 				return 0;
	703a711,712
	> 			if(isdotdot(field[0]))
	> 				return 0;
	721a731,736
	> }
	> 
	> static int
	> isdotdot(char *n)
	> {
	> 	return n[0]=='.' && (n[1]=='\0' || n[1]=='.' && n[2]=='\0');

I think this is easier and more correct since it covers all systems, not
just Unix-like:

	at the end of the switch statement of crackdir(), before
	line 714, add

	if(strcmp(longname, ".") == 0 || strcmp(longname, "..") == 0)
		return 0;

or, if the strcmp's bug you, use forsyth's isdotdot().