use intptr_t for file descriptor abstraction so that Win32 HANDLEs can be retained; fix other warnings too

diff --git a/include/picotls.h b/include/picotls.h
index fb6567d..2bdd490 100644
--- a/include/picotls.h
+++ b/include/picotls.h

@@ -779,7 +779,7 @@
     /**
      * optional callback returning a file descriptor that becomes readable when the job is complete
      */
-    int (*get_fd)(struct st_ptls_async_job_t *self);
+    intptr_t (*get_fd)(struct st_ptls_async_job_t *self);
     /**
      * optional callback for setting a completion callback
      */

diff --git a/lib/openssl.c b/lib/openssl.c
index 19b5cfe..3ed9c15 100644
--- a/lib/openssl.c
+++ b/lib/openssl.c

@@ -1088,7 +1088,7 @@
     free(self);
 }
 
-int async_sign_ctx_get_fd(ptls_async_job_t *_self)
+intptr_t async_sign_ctx_get_fd(ptls_async_job_t *_self)
 {
     struct async_sign_ctx *self = (void *)_self;
     OSSL_ASYNC_FD fds[1];
@@ -1097,7 +1097,7 @@
     ASYNC_WAIT_CTX_get_all_fds(self->waitctx, NULL, &numfds);
     assert(numfds == 1);
     ASYNC_WAIT_CTX_get_all_fds(self->waitctx, fds, &numfds);
-    return (int)fds[0];
+    return (intptr_t)fds[0];
 }
 
 static ptls_async_job_t *async_sign_ctx_new(const ptls_openssl_signature_scheme_t *scheme, EVP_MD_CTX *ctx, size_t siglen)

diff --git a/t/openssl.c b/t/openssl.c
index 1380548..638503e 100644
--- a/t/openssl.c
+++ b/t/openssl.c

@@ -482,7 +482,7 @@
             case PTLS_ERROR_ASYNC_OPERATION: {
                 ptls_async_job_t *job = ptls_get_async_job(qat.conns[offending].tls);
                 assert(job->get_fd != NULL);
-                qat.conns[offending].wait_fd = job->get_fd(job);
+                qat.conns[offending].wait_fd = (int)job->get_fd(job);
                 assert(qat.conns[offending].wait_fd != -1);
             } break;
             default:

diff --git a/t/picotls.c b/t/picotls.c
index 799b803..2200d23 100644
--- a/t/picotls.c
+++ b/t/picotls.c

@@ -1220,7 +1220,7 @@
             int ret = sign_certificate(self, tls, NULL, selected_algorithm, &fakebuf, input, algorithms, num_algorithms);
             assert(ret == 0);
             ptls_buffer_dispose(&fakebuf);
-            async_ctx.super.destroy_ = (void (*)(ptls_async_job_t *))0xdeadbeef;
+            async_ctx.super.destroy_ = NULL;
             async_ctx.selected_algorithm = *selected_algorithm;
             *async = &async_ctx.super;
             --server_sc_callcnt;