Implement $UID and $EUID (a bash extension)

Addresses issue #37.

core/state.py

@@ -242,6 +242,10 @@ def _InitDefaults(self):
    SetGlobalString(self, 'IFS', ' \t\n')
    SetGlobalString(self, 'PWD', os.getcwd())

    # NOTE: Should we put these in a namespace for Oil?
    SetGlobalString(self, 'UID', str(os.getuid()))
    SetGlobalString(self, 'EUID', str(os.geteuid()))

  def _InitEnviron(self, environ):
    # This is the way dash and bash work -- at startup, they turn everything in
    # 'environ' variable into shell variables.  Bash has an export_env

spec/special-vars.test.sh

@@ -98,3 +98,13 @@ expr $0 : '.*/osh$' && exit 99  # Disabled because of spec-runner.sh issue
echo $RANDOM | egrep '[0-9]+'
# status: 0
# N-I dash status: 1

### $UID and $EUID
# These are both bash-specific.
set -o errexit
echo $UID | egrep -o '[0-9]+' >/dev/null
echo $EUID | egrep -o '[0-9]+' >/dev/null
echo status=$?
# stdout: status=0
# N-I dash/mksh stdout-json: ""
# N-I dash/mksh status: 1