junos: Introduced junos_invoke_method().

This function may be used to call an arbitrary JUNOScript method with any
number and type of attributes and arguments. Attributes and arguments are
specified using a variable argument list using tuples <type>, <name>[,
<value>] where type is any of a list of constants defined in junos.h. The type
of <value> is determined from the specified type. The list has to be
terminated by the constant JUNOS_NO_ARGS.

junos_simple_method() has been "degraded" to a define calling
junos_invoke_method().