allow $fu/bar and don't choke on files named "-".

Reference: /n/atom/patch/applied/mkfsdollar
Date: Sat Feb 22 17:02:00 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/disk/mkfs.c	Sat Feb 22 17:00:53 2014
+++ /sys/src/cmd/disk/mkfs.c	Sat Feb 22 17:00:54 2014
@@ -610,10 +610,11 @@
 	p = getname(p, &f->gid);
 	if(!*f->gid)
 		f->gid = "-";
-	f->old = getpath(p);
-	if(f->old && strcmp(f->old, "-") == 0){
+	f->old = nil;
+	getname(p, &f->old);
+	if(f->old != nil && (f->old[0] == 0 || strcmp(f->old, "-") == 0)){
 		free(f->old);
-		f->old = 0;
+		f->old = nil;
 	}
 	setnames(f);
 
@@ -644,6 +645,25 @@
 }
 
 char*
+getenv1(char *s)
+{
+	char *p, *env, *r;
+
+	p = strchr(s, '/');
+	if(p != nil)
+		*p++ = 0;
+	env = getenv(s);
+	if(env == nil)
+		return nil;
+	if(p){
+		r = smprint("%s/%s", env, p);
+		free(env);
+	}else
+		r = env;
+	return r;
+}
+
+char*
 getname(char *p, char **buf)
 {
 	char *s, *start;
@@ -661,11 +681,12 @@
 	(*buf)[p-start] = '\0';
 
 	if(**buf == '$'){
-		s = getenv(*buf+1);
+		s = getenv1(*buf+1);
 		if(s == 0){
 			warn("can't read environment variable %q", *buf+1);
 			skipdir();
 			free(*buf);
+			*buf = nil;
 			return nil;
 		}
 		free(*buf);