issue #277: core: move Darwin versioner check into first stage

The 'versioner.c' dodging check added in 0ef23d86 was wrong, since the
check occurred on the host machine, when the fix actually needs to apply
to the Darwin target.

Fixes ability to target OS X from a Red Hat controller, manifesting as
an error like:

    D mitogen: mitogen.parent.TtyLogStream('local.2472'):  'python(mitogen:dmw@localhost.localdomain:2449): realpath couldn\'t resolve "/usr/bin/python(mitogen:dmw@localhost.localdomain:2449)"'

The "realpath couldn't resolve" error comes from versioner.c:

    https://opensource.apple.com/source/perl/perl-104/versioner/versioner.c

mitogen/parent.py

@@ -686,11 +686,6 @@ class Stream(mitogen.core.Stream):
         self.max_message_size = max_message_size
         if python_path:
             self.python_path = python_path
-        if sys.platform == 'darwin' and self.python_path == '/usr/bin/python':
-            # OS X installs a craptacular argv0-introspecting Python version
-            # switcher as /usr/bin/python. Override attempts to call it with an
-            # explicit call to python2.7
-            self.python_path = '/usr/bin/python2.7'
         if connect_timeout:
             self.connect_timeout = connect_timeout
         if remote_name is None:
@@ -775,6 +770,9 @@ class Stream(mitogen.core.Stream):
     #     substituted with their respective values.
     #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
     #     order to allow virtualenvs to detect their install prefix.
+    #   * For Darwin, OS X installs a craptacular argv0-introspecting Python
+    #     version switcher as /usr/bin/python. Override attempts to call it
+    #     with an explicit call to python2.7
     @staticmethod
     def _first_stage():
         R,W=os.pipe()
@@ -787,6 +785,8 @@ class Stream(mitogen.core.Stream):
             os.close(r)
             os.close(W)
             os.close(w)
+            if sys.platform == 'darwin' and sys.executable == '/usr/bin/python':
+                sys.executable += sys.version[:3]
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'EC0\n')

mitogen/ssh.py

@@ -91,7 +91,6 @@ class HostKeyError(mitogen.core.StreamError):
 class Stream(mitogen.parent.Stream):
     create_child = staticmethod(mitogen.parent.hybrid_tty_create_child)
     child_is_immediate_subprocess = False
-    python_path = 'python2.7'
 
     #: Number of -v invocations to pass on command line.
     ssh_debug_level = 0