Make COMP_CTX and COMP_METHOD opaque

Since COMP_METHOD is now defined in comp_lcl.h, it is no
longer possible to create new TLS compression methods without
using the OpenSSL source.  Only ZLIB is supported by default.
Also, since the types are opaque, #ifdef guards to use "char *"
instead of the real type aren't necessary.

The changes are actually minor.  Adding missing copyright to some
files makes the diff misleadingly big.

Reviewed-by: Matt Caswell <matt@openssl.org>