Fix errno not defined [closes GH-465]

lib/vagrant/util/safe_exec.rb

@@ -8,13 +8,20 @@ module Vagrant
     # forking.
     module SafeExec
       def safe_exec(command)
+        # Create a list of things to rescue from. Since this is OS
+        # specific, we need to do some defined? checks here to make
+        # sure they exist.
+        rescue_from = []
+        rescue_from << Errno::EOPNOTSUPP if defined?(Errno::EOPNOTSUPP)
+        rescue_from << Errno::E045 if defined?(Errno::E045)
+
         fork_instead = false
         begin
           pid = nil
           pid = fork if fork_instead
           Kernel.exec(command) if pid.nil?
           Process.wait(pid) if pid
-        rescue Errno::E045
+        rescue *rescue_from
           # We retried already, raise the issue and be done
           raise if fork_instead