context: for dhparam_cb, pass is_export as boolean

The integer value that's actually returned for this flag is 2, which is fine
for C (it is defined as true), but it's sufficiently surprising (because it's
not 1), that this is worth fixing -- even if export ciphers aren't common.

It should be a boolean anyway.
context: Link SSL_CTX to p_context (not lua_State)

This is needed because the p_context is going to cache DH (and eventually
EC_KEY) objects, to plug a leak in the dhparam callback.
context: Don't leak DH* in dhparam_cb

==1429== 336 (144 direct, 192 indirect) bytes in 1 blocks are definitely lost in loss record 567 of 611
...
==1429==    by 0x5ECCBC7: PEM_ASN1_read_bio (in /lib/x86_64-linux-gnu/libcrypto.so.1.0.0)
==1429==    by 0x4E39D8F: dhparam_cb (context.c:184)
==1429==    by 0x5B679D3: ??? (in /lib/x86_64-linux-gnu/libssl.so.1.0.0)
==1429==    by 0x5B6A6EE: ??? (in /lib/x86_64-linux-gnu/libssl.so.1.0.0)
==1429==    by 0x4E3C00D: meth_handshake (ssl.c:103)
...