Handle .do files that start with "#!/" to specify an explicit interpreter.

Now you can have your .do files interpreted by whatever interpreter you
want.

t/.gitignore

@@ -23,3 +23,4 @@ test2.args
 /ifcreate[12]
 /broken
 /shellfile
+/nonshelltest

t/clean.do

@@ -1,6 +1,6 @@
 redo example/clean curse/clean deps/clean "space dir/clean" stamp/clean \
 	defaults-flat/clean
-rm -f broken shellfile mode1 makedir.log chdir1 deltest2 \
+rm -f broken nonshelltest shellfile mode1 makedir.log chdir1 deltest2 \
 	hello [by]ellow *.o *~ .*~ *.log CC LD passfail silence silence.do \
 	touch1 touch1.do always1 ifcreate[12].dep ifcreate[12]
 rm -rf makedir

t/nonshelltest.do

@@ -0,0 +1,3 @@
+#!/usr/bin/env perl
+$a="perly";
+print "hello $a world\n";

t/test.do

@@ -1,5 +1,6 @@
 redo-ifchange all
 ./hello >&2
-redo shelltest deltest deltest2 test.args test2.args passfailtest chdirtest \
+redo nonshelltest shelltest \
+	deltest deltest2 test.args test2.args passfailtest chdirtest \
 	curse/test deps/test "space dir/test" modetest makedir2 \
 	silencetest touchtest stamp/test alwaystest ifcreate-test