Use tls_buffer for alert and handshake fragments in the legacy stack.

This avoids a bunch of pointer munging and a handrolled memmove.

ok tb@

Add read and write support to tls_buffer.

tls_buffer was original created for a specific use case, namely reading in
length prefixed messages. This adds read and write support, along with a
capacity limit, allowing it to be used in additional use cases.

ok beck@ tb@

Remove tls_buffer_set_data() and remove/revise callers.

There is no way that tls_buffer_set_data() can currently work in
conjunction with tls_buffer_expand(). This fact is currently hidden by the
way that PHH works, which reads the same data from the record layer (which
it needs to do anyway, since we may not have all of the handshake message
in a single record).

Since this is broken, mop it up and change the PHH callback to not provide
the record data.

ok beck@ tb@

Rename tls13_buffer to tls_buffer.

This code will soon be used in the DTLSv1.2 and TLSv1.2 stack. Also
introduce tls_internal.h and move/rename the read/write/flush callbacks.

ok beck@ tb@