sh: Set $? to 0 for background commands.

For backgrounded pipelines and subshells, the previous value of $? was being
preserved, which is incorrect.

For backgrounded simple commands containing a command substitution, the
status of the last command substitution was returned instead of 0.

If fork() fails, this is an error.

bin/sh/eval.c

@@ -420,7 +420,8 @@ evalsubshell(union node *n, int flags)
 		INTOFF;
 		exitstatus = waitforjob(jp, (int *)NULL);
 		INTON;
-	}
+	} else
+		exitstatus = 0;
 }
 
 
@@ -559,7 +560,8 @@ evalpipe(union node *n)
 		exitstatus = waitforjob(jp, (int *)NULL);
 		TRACE(("evalpipe:  job done exit status %d\n", exitstatus));
 		INTON;
-	}
+	} else
+		exitstatus = 0;
 }
 
 
@@ -1056,7 +1058,8 @@ evalcommand(union node *cmd, int flags, struct backcmd *backcmd)
 		backcmd->fd = pip[0];
 		close(pip[1]);
 		backcmd->jp = jp;
-	}
+	} else
+		exitstatus = 0;
 
 out:
 	if (lastarg)

tools/regression/bin/sh/execution/bg1.0

@@ -0,0 +1,3 @@
+# $FreeBSD$
+
+: `false` &

tools/regression/bin/sh/execution/bg2.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+f() { return 42; }
+f
+: | : &

tools/regression/bin/sh/execution/bg3.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+f() { return 42; }
+f
+(:) &