Support custom URI schemes for the keylocation property

Every platform has their own preferred methods for implementing URI 
schemes beyond the currently supported file scheme (e.g. 'https' on 
FreeBSD would likely use libfetch, while Linux distros and illumos
would probably use libcurl, etc). It would be helpful if libzfs can 
be extended to support additional schemes in a simple manner.

A table of (scheme, handler_function) pairs is added to libzfs_crypto.c, 
and the existing functions in libzfs_crypto.c so that when the key 
format is ZFS_KEYFORMAT_URI, the scheme from the URI string is 
extracted, and a matching handler it located in the aforementioned 
table (returning an error if no matching handler is found). The handler 
function is then invoked to retrieve the key material (in the format 
specified by the keyformat property) and the key is loaded or the 
handler can return an error to abort the key loading process.

Reviewed by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jason King <jason.king@joyent.com>
Closes #10218

include/libzfs_impl.h

@@ -23,6 +23,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011, 2020 by Delphix. All rights reserved.
  * Copyright (c) 2018 Datto Inc.
+ * Copyright 2020 Joyent, Inc.
  */
 
 #ifndef	_LIBZFS_IMPL_H
@@ -33,6 +34,7 @@
 #include <sys/nvpair.h>
 #include <sys/dmu.h>
 #include <sys/zfs_ioctl.h>
+#include <regex.h>
 
 #include <libuutil.h>
 #include <libzfs.h>
@@ -71,6 +73,7 @@ struct libzfs_handle {
 	int libzfs_pool_iter;
 	char libzfs_chassis_id[256];
 	boolean_t libzfs_prop_debug;
+	regex_t libzfs_urire;
 };
 
 #define	ZFSSHARE_MISS	0x01	/* Didn't find entry in cache */
@@ -124,6 +127,14 @@ typedef enum {
 	SHARED_SMB = 0x4
 } zfs_share_type_t;
 
+typedef int (*zfs_uri_handler_fn_t)(struct libzfs_handle *, const char *,
+    const char *, zfs_keyformat_t, boolean_t, uint8_t **, size_t *);
+
+typedef struct zfs_uri_handler {
+	const char *zuh_scheme;
+	zfs_uri_handler_fn_t zuh_handler;
+} zfs_uri_handler_t;
+
 #define	CONFIG_BUF_MINSIZE	262144
 
 int zfs_error(libzfs_handle_t *, int, const char *);