Rename initial_ctx to session_ctx.

This makes its purpose clearer. That the session cache is based on the
initial SSL_CTX is confusing (it's a remnant of OpenSSL's backwards
session resumption ordering), but we're probably stuck with it.
Relatedly, document SSL_set_SSL_CTX better.

Change-Id: I2832efc63f6c959c5424271b365825afc7eec5e4
Reviewed-on: https://boringssl-review.googlesource.com/14204
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 4a7fbd3..34478ed 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -3013,9 +3013,9 @@
   if (ssl->ctx->tlsext_servername_callback != 0) {
     ret = ssl->ctx->tlsext_servername_callback(ssl, &al,
                                                ssl->ctx->tlsext_servername_arg);
-  } else if (ssl->initial_ctx->tlsext_servername_callback != 0) {
-    ret = ssl->initial_ctx->tlsext_servername_callback(
-        ssl, &al, ssl->initial_ctx->tlsext_servername_arg);
+  } else if (ssl->session_ctx->tlsext_servername_callback != 0) {
+    ret = ssl->session_ctx->tlsext_servername_callback(
+        ssl, &al, ssl->session_ctx->tlsext_servername_arg);
   }
 
   switch (ret) {
@@ -3048,7 +3048,7 @@
                        size_t ticket_len, const uint8_t *session_id,
                        size_t session_id_len) {
   int ret = 1; /* Most errors are non-fatal. */
-  SSL_CTX *ssl_ctx = ssl->initial_ctx;
+  SSL_CTX *ssl_ctx = ssl->session_ctx;
   uint8_t *plaintext = NULL;
 
   HMAC_CTX hmac_ctx;