This commit introduces source files
- `ssl_12_gen.c`
- `ssl_12_srv.c`
- `ssl_12_cli.c`
which will subsequently be populated with implementations
for handshake reading and writing functions for all TLS
versions <= 1.2. `ssl_12_gen.c` shall contain functions
shared between client and server, while `ssl_12_cli.c`
and `ssl_12_srv.c` shall contain client/server-specific
functions only.
In order to retain git history, those files are so far
identical copies of `ssl_tls.c`, `ssl_srv.c` and `ssl_cli.c`,
and those latter files have been moved to temporary files
`ssl_xxx_old.c`. Subsequent commits will rename them back
to their original name and do the actual code split.
