Adapt to RegGetValue brokenness in Windows 7

Adapt to RegGetValue brokenness in Windows 7

- RegGetValue with flags = RRF_RT_REG_SZ|RRF_RT_REG_EXPAND_SZ
  fails in Windows 7 with an "invalid parameter" error.
  Fix by using RRF_RT_REG_SZ alone.

Note: This is not a regression as in no released version did the
service support expandable strings (ones with embedded %FOO%) in
the registry. However, the GUI does expand such strings. The two
can be made consistent by explicitly expanding the strings -- that
is left for a future patch.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1519191510-3826-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16513.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>