getflags: import from 4e, with usage

fixes #6 http://bitbucket.org/rsc/plan9port/issue/6/

http://codereview.appspot.com/95043

cmd/usage.c

@@ -0,0 +1,72 @@
+#include <u.h>
+#include <libc.h>
+
+void
+main(int argc, char **argv)
+{
+	Fmt fmt;
+	char buf[512];
+	char *argv0, *args, *flags, *p, *p0;
+	int single;
+	Rune r;
+	
+	argv0 = getenv("0");
+	if(argv0 == nil) {
+		if(argc > 1)
+			argv0 = argv[1];
+		else
+			argv0 = "unknown-program-name";
+	}
+	if((p = strrchr(argv0, '/')) != nil)
+		argv0 = p+1;
+	flags = getenv("flagfmt");
+	args = getenv("args");
+	
+	if(argv0 == nil){
+		fprint(2, "aux/usage: $0 not set\n");
+		exits("$0");
+	}
+	if(flags == nil)
+		flags = "";
+	if(args == nil)
+		args = "";
+
+	fmtfdinit(&fmt, 2, buf, sizeof buf);
+	fmtprint(&fmt, "usage: %s", argv0);
+	if(flags[0]){
+		single = 0;
+		for(p=flags; *p; ){
+			p += chartorune(&r, p);
+			if(*p == ',' || *p == 0){
+				if(!single){
+					fmtprint(&fmt, " [-");
+					single = 1;
+				}
+				fmtprint(&fmt, "%C", r);
+				if(*p == ',')
+					p++;
+				continue;
+			}
+			while(*p == ' ')
+				p++;
+			if(single){
+				fmtprint(&fmt, "]");
+				single = 0;
+			}
+			p0 = p;
+			p = strchr(p0, ',');
+			if(p == nil)
+				p = "";
+			else
+				*p++ = 0;
+			fmtprint(&fmt, " [-%C %s]", r, p0);
+		}
+		if(single)
+			fmtprint(&fmt, "]");
+	}
+	if(args)
+		fmtprint(&fmt, " %s", args);
+	fmtprint(&fmt, "\n");
+	fmtfdflush(&fmt);
+	exits("usage");
+}