daemon: Make gdisk into an optional dependency and optgroup.

Also document that gdisk is a dependency at all.

This fixes commit 956e30effa.

README

@@ -118,6 +118,9 @@ For basic functionality and the C tools:
 - yajl >= 2 (optional)
   JSON parser, needed to handle the output of ldmtool.
 
+- gdisk (optional)
+  For managing some aspects of GPT disks.
+
 - netpbm, icoutils (optional)
   These programs are used to render icons from guests.
 

daemon/parted.c

@@ -27,6 +27,7 @@
 
 #include "daemon.h"
 #include "actions.h"
+#include "optgroups.h"
 
 GUESTFSD_EXT_CMD(str_parted, parted);
 GUESTFSD_EXT_CMD(str_sfdisk, sfdisk);
@@ -804,6 +805,12 @@ do_part_set_mbr_id (const char *device, int partnum, int idbyte)
   return 0;
 }
 
+int
+optgroup_gdisk_available (void)
+{
+  return prog_exists (str_sgdisk);
+}
+
 int
 do_part_set_gpt_type(const char *device, int partnum, const char *guid)
 {

generator/actions.ml

@@ -10650,6 +10650,7 @@ group with GUID <diskgroup>." };
     name = "part_set_gpt_type";
     style = RErr, [Device "device"; Int "partnum"; String "guid"], [];
     proc_nr = Some 392;
+    optional = Some "gdisk";
     tests = [];
     shortdesc = "set the type GUID of a GPT partition";
     longdesc = "\
@@ -10664,6 +10665,7 @@ for a useful list of type GUIDs." };
     name = "part_get_gpt_type";
     style = RString "guid", [Device "device"; Int "partnum"], [];
     proc_nr = Some 393;
+    optional = Some "gdisk";
     tests = [
       InitGPT, Always, TestOutput (
         [["part_set_gpt_type"; "/dev/sda"; "1";