Add support for AWE32 NRPNs

[derselbst]

This adds a compatibility layer for AWE32-style NRPNs, see #1342.

Essentially, it adds a few more members to fluid_chan_t, allowing us to memorize any generators overridden by AWE32 NRPNs.

fluid_hz2ct() has been revived and fluid_sec2tc() has been added.

And fluid_mod_transform_source_value() is now extern.