Add cygsid methods to create SIDs from scratch

So far creating cygsids requires to generate an "S-1-..." string
which is then converted to a SID by cygsid::getfromstr.

Add two new methods:

- cygsid::create (DWORD auth, DWORD subauth_count, ...)

    ... is a variable length list of subauth_count DWORD values being
    the actual subauths.

- cygsid::append (DWORD rid)

    allows to append a single RID to an alreaday constituted SID.

	* security.h (cygsid::create): Declare public.
	(cygsid::append): Ditto.
	* sec_helper.cc (cygsid::create): Implement.
	(cygsid::append): Implement.
	* uinfo.cc (pwdgrp::fetch_account_from_windows): Use both new
	methods as appropriate.  Drop setting csid from string.  Create
	SID strings for printing SIDs only.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/sec_helper.cc

@@ -13,6 +13,7 @@ details. */
 
 #include "winsup.h"
 #include <stdlib.h>
+#include <stdarg.h>
 #include <cygwin/acl.h>
 #include <sys/queue.h>
 #include <authz.h>
@@ -284,6 +285,37 @@ cygsid::getfromstr (const char *nsidstr, bool well_known)
   return psid = NO_SID;
 }
 
+const PSID
+cygsid::create (DWORD auth, DWORD subauth_cnt, ...)
+{
+  va_list ap;
+  PSID sid;
+
+  if (subauth_cnt > SID_MAX_SUB_AUTHORITIES)
+    return NULL;
+
+  DWORD subauth[subauth_cnt];
+
+  va_start (ap, subauth_cnt);
+  for (DWORD i = 0; i < subauth_cnt; ++i)
+    subauth[i] = va_arg (ap, DWORD);
+  sid = get_sid (auth, subauth_cnt, subauth, false);
+  va_end (ap);
+  return sid;
+}
+
+bool
+cygsid::append (DWORD rid)
+{
+  if (psid == NO_SID)
+    return false;
+  PISID dsid = (PISID) psid;
+  if (dsid->SubAuthorityCount >= SID_MAX_SUB_AUTHORITIES)
+    return false;
+  dsid->SubAuthority[dsid->SubAuthorityCount++] = rid;
+  return true;
+}
+
 cygsid *
 cygsidlist::alloc_sids (int n)
 {