Add support for changing the loglevels for given modules (EJAB-225)

Add support for changing the loglevels for given modules (EJAB-225)

ejabberd_loglevel:set/1 now understands more input formats:

set(Param) -> {module, ejabberd_logger}
    Param = Level | {Level, CustomLevels}
    CustomLevels = [CustomLevel]
    CustomLevel = {Module, Level}
    Module = atom()
    Level = integer() | atom()

set(Level) is equivalent to set({Level, []}), so it clears all module
level customizations. log_src/2 adds additional function heads for those
_msg function and module combinations that need non-default behavior,
i.e. add or filter logging.

ejabberd_loglevel:get/0 only returns the default log level. To retrieve
the complete setting use ejabberd_logger:get/0 instead.