sh: Reduce unnecessary forks with eval.

The eval special builtin now runs the code with EV_EXIT if it was run
with EV_EXIT itself.

In particular, this eliminates one fork when a command substitution contains
an eval command that ends with an external program or a subshell.

This is similar to what r220978 did for functions.

bin/sh/eval.c

@@ -140,7 +140,7 @@ evalcmd(int argc, char **argv)
                         STPUTC('\0', concat);
                         p = grabstackstr(concat);
                 }
-                evalstring(p, builtin_flags & EV_TESTED);
+                evalstring(p, builtin_flags);
         } else
                 exitstatus = 0;
         return exitstatus;
@@ -908,6 +908,7 @@ evalcommand(union node *cmd, int flags, struct backcmd *backcmd)
 				dup2(pip[1], 1);
 				close(pip[1]);
 			}
+			flags &= ~EV_BACKCMD;
 		}
 		flags |= EV_EXIT;
 	}

tools/regression/bin/sh/expansion/cmdsubst11.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+# Not required by POSIX but useful for efficiency.
+
+[ $$ = $(eval '${SH} -c echo\ \$PPID') ]