Rename debug to ssl_debug + make it internal

[mpg]

The debug module, currently debug.h + debug.c is not about generic debugging but actually part of SSL. It's the only part of SSL that doesn't start with ssl_, so this should be fixed by renaming to ssl_debug. (Once this is done, all parts of libmbedtls.a will start with ssl_ and all parts of libmbedx509.a with x509_, so once crypto is split out everything will be neat and tidy.)

Also, it's unclear is most of whats currently in debug.h is really meant as a public API for applications to use, or only for internal use by the library. IMO only mbedtls_debug_set_threshold() should really be public and everything else internal. If we go this way, then we might want to move its declaration to ssl.h (and rename to mbedtls_ssl_debug_set_threshold() while at it), and then have no public ssl_debug.h, just an internal one in library.

Similarly MBEDTLS_DEBUG_C should be renamed MBEDTLS_SSL_DEBUG_C for consistency.