Skipping most of the PLC code for DTX

Also cap the encoder bit-rate to 260 kb/s per channel.
1275 byte packets is the absolute maximum packet size we're going to support. This makes the encoder and decoder refuse to operate outside of these boundaries.
Configure option for custom modes
STATIC_MODES no longer used
Bump LT_CURRENT and fix configure output messages
Better handling of the bandwidth
Tuning the hybrid bit-rate split
Making test_opus default to VBR
oops
Frame size optional, defaults to 20 ms
CELT update
Making encoder-side clipping optional
draft date
Fix test_opus args count
Remove opus_enc and opus_dec from the build
Allowing maxCompressedBytes to be as large as one wants

Though the encoder will still never use more than 1275 bytes
Fix previous commit by still capping what the encoder will produce
16-bit int fixes.

This fixes a number of issues for platforms with a 16-bit int, but
 by no means all of them.
The type change for ec_window (for platforms where sizeof(size_t)==2)
 will break ABI (but not API) compatibility with libsilk and libopus,
 and reduce speed on x86-64, but allows the code to work in real-mode
 DOS without using the huge memory model, which is useful for testing
 16-bit int compliance.
minor detail with the last patch
Update celtclient to the new API.