GH787: Fix ALPN

* Perform ALPN after the SNI callback; the SSL_CTX may change due to
  that processing
* Add flags to indicate that we actually sent ALPN, to properly error
  out if unexpectedly received.
* clean up ssl3_free() no need to explicitly clear when doing memset
* document ALPN functions

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>

apps/apps.c

diff --git a/apps/apps.c b/apps/apps.c
index 19523d6..4e2322d 100644
--- a/apps/apps.c
+++ b/apps/apps.c
@@ -1960,7 +1960,7 @@
  *
  *   returns: a malloced buffer or NULL on failure.
  */
-unsigned char *next_protos_parse(unsigned short *outlen, const char *in)
+unsigned char *next_protos_parse(size_t *outlen, const char *in)
 {
     size_t len;
     unsigned char *out;