bug#39660: openvpn-client-service does not support auth-user-pass

[Joshua Branson via Bug reports for GNU Guix <bug-guix@gnu.org>]

Julien and I discussed on irc that guix currently does not have a
method of generating my config file.  Here is just an updated list of
the options that I (and possibly others) may need or want.

#+BEGIN_SRC org
These are all the options that my config file has.  If the box does
not have an X, then we should add this in the service definition.

- [ ] "persist-key"
- [ ] "persist-tun"
- [ ] "remote-random"
- [ ] "pull"
- [X] "comp-lzo no"
- [ ] "tls-client"  does tls-auth provide this option???
- [ ] "verify-x509-name Server name-prefix"
- [ ] "ns-cert-type server"  This is possibly deprecated?
- [ ] "key-direction 1" This is another way of specifying tls-auth?
- [X] "route-method exe" This is only useful on Windows.
- [ ] "route-delay 2"
- [X] "tun-mtu 1500" The documentation says most cases...I should
  leave this to it's default parameter.  So unless needed, we probably
  shouldn't need to add it to guix.
  
- The next two options only make sense when we are using the protocol
  udp.  We should probably specify them someway that you can only use
  them if protocol is upd.  Something like:

   #+BEGIN_SRC scheme
   (proto udp
     (upd-options
       (fragment 1300)
       (mssfix 1200))
   #+END_SRC

- [X] "fragment 1300"
- [X] "mssfix 1200"


- [ ] "cipher AES-256-CBC"
- [X] keysize 256 deprecated.  Do not need. and my key size is the
  cipher size anyway.  The documentation does not reccommend manually changing your keysize.
- [X] auth SHA512  I have no idea where this is in the documentation
- [X] sndbuf 524288  The documentation says that the default should work.
- [X] rcvbuf 524288  as above
- [X] auth-user-pass login.conf
#+END_SRC

We should also probably allow a file option.  Some users may have a
working file.  Perhaps we should support this:

#+BEGIN_SRC scheme
(openvpn-client-service
  #:file  "/path/to/openvpn.conf")
#+END_SRC

Joshua