Add CUser::LoadModule() helper

Makes it easier to retry loading replacement modules when
loading retired modules fail.
Retire awaynick (resolves #661)

== built-in awaynick ==

debug output on startup:

    [ .. ] Loading network module [awaynick]...
    [ !! ] Module [awaynick] aborted: retired module - see http://wiki.znc.in/awaynick
    [ .. ] Loading network module [simple_away] instead...
    [ >> ] [/Users/jpnurmi/znc/lib/znc/simple_away.so]

loadmod client command:

    <jpnurmi> loadmod awaynick
    <*status> Module [awaynick] aborted: retired module - see http://wiki.znc.in/awaynick

webadmin description:

    retired module - see http://wiki.znc.in/awaynick

webadmin load attempt:

    Unable to load module [awaynick] [Module [awaynick] aborted:
    retired module - see http://wiki.znc.in/awaynick]

== external awaynick ==

znc-buildmod:

    $ znc-buildmod awaynick.cpp
    Building "awaynick.so" for ZNC 1.5... [ ok ]

debug output on startup:

    [ .. ] Loading network module [awaynick]...
    [ >> ] [/Users/jpnurmi/.znc/modules/awaynick.so]

loadmod client command:

    <jpnurmi> loadmod awaynick
    <*status> Loaded module [awaynick] [/Users/jpnurmi/.znc/modules/awaynick.so]

webadmin description:

    Change your nick while you are away