Reflect OPENSSL_NO_SOCK and OPENSSL_NO_POSIX_IO into headers

Like OPENSSL_NO_FILESYSTEM, keep us honest: if the symbol is missing,
don't declare it in the headers. This ensures folks aren't relying on
dead code elimination and then later break when they build in a context
where it doesn't happen.

Bug: 629
Change-Id: I3e56c3879e970aa8d0d6e0e5f1ad046d0f420ef0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61730
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 26ffd50..5a2ac2a 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -1595,6 +1595,7 @@
   return ret;
 }
 
+#if !defined(OPENSSL_NO_SOCK)
 int SSL_set_fd(SSL *ssl, int fd) {
   BIO *bio = BIO_new(BIO_s_socket());
   if (bio == NULL) {
@@ -1644,6 +1645,7 @@
   }
   return 1;
 }
+#endif  // !OPENSSL_NO_SOCK
 
 static size_t copy_finished(void *out, size_t out_len, const uint8_t *in,
                             size_t in_len) {