fix quotes in --altsvc example

Fix decoder table size update

Remove SPDY option for CMake

Update nghttp2.pyx

When applying new header table size acknowledged with SETTINGS ACK by
an encoder, change the header table size on a decoder only when it
strictly lowers the current maximum table size set by Dynamic Table
Size Update from the encoder or the default size 4096 if no Dynamic
Table Size Update is received.

Previously, the header table size on a decoder is always changed.  If
a maximum size in SETTINGS are increased (e.g., 4096 -> 8192), and
then decreased to the previous value, the decoder incorrectly requires
Dynamic Table Size Update from an encoder.

SPDY feature removed in #1091 and release v1.29.0

Make Docker speak HTTP/3

... if HTTP/3 support is built in.