Merge pull request #19 from blueyed/do-not-close-fds

Keep file descriptors with Popen (for `is_windows`)
Change `isenvvar()` to only check for `=` in var names.

Daemontools' envdir only checks for `=`, thus this python envdir version
should have the same behavior.
Merge pull request #24 from hltbra/master

Change `isenvvar()` to only check for `=` in var names
Add setsignals and call it for shell and run only

The signals must not get setup in __init__ when envdir is used via the
Python API.

Fixes #21
Merge branches 'do-not-close-fds' and 'fix-kill'
Merge branch 'setsignals'
Merge remote-tracking branch 'blueyed/master'