lib9, cmd/dist, cmd/ld: Plan 9: fix build

lib9: fix runcmd, removeall, and tempdir functions

cmd/dist: Include run_plan9.c and tempdir_plan9.c
        from lib9 for build, and in general consider
        file names containing "plan9" for building.

cmd/ld: provide function args for the new functions
        from lib9.

R=rsc, rminnich, ality, bradfitz
CC=golang-dev
https://golang.org/cl/7666043

src/cmd/dist/build.c

@@ -1070,7 +1070,8 @@ shouldbuild(char *file, char *dir)
 			name = lastelem(file);
 			if(streq(name, "goos.c") || streq(name, "flag.c"))
 				return 1;
-			return 0;
+			if(!contains(name, "plan9"))
+				return 0;
 		}
 		if(streq(dir, "libbio"))
 			return 0;

src/cmd/ld/lib.c

@@ -556,6 +556,11 @@ hostobjs(void)
 	}
 }
 
+// provided by lib9
+int runcmd(char**);
+char* mktempdir(void);
+void removeall(char*);
+
 static void
 rmtemp(void)
 {

src/lib9/run_plan9.c

@@ -15,9 +15,9 @@ runcmd(char **argv)
 	case -1:
 		return -1;
 	case 0:
-		execvp(argv[0], argv);
+		exec(argv[0], argv);
 		fprint(2, "exec %s: %r", argv[0]);
-		_exit(1);
+		exits("exec");
 	}
 	
 	w = wait();

src/lib9/tempdir_plan9.c

@@ -9,7 +9,7 @@ char*
 mktempdir(void)
 {
 	char *p;
-	int i;
+	int fd, i;
 	
 	p = smprint("/tmp/go-link-XXXXXX");
 	for(i=0; i<1000; i++) {
@@ -27,7 +27,7 @@ mktempdir(void)
 void
 removeall(char *p)
 {
-	int fd, n;
+	int fd, n, i;
 	Dir *d;
 	char *q;