Cygwin: passwd/group: store account name case correct

When looking up valid accounts by name, LookupAccountName returns
a SID and a case-correct domain name.  However, the name was input
and LookupAccountName is case-insensitive, so the name is not
necessarily written the same way as in SAM or AD.

Fix that by doing a reverse lookup on the just fetched SID.  This
fetches the account name in the correct case.  Override the
incoming name with the case correct name from LookupAccountSid.

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

winsup/cygwin/release/3.0

@@ -109,3 +109,7 @@ Bug Fixes
 
 - Fix O_APPEND handling on files when calling non-Cygwin applications
   Addresses: https://cygwin.com/ml/cygwin/2019-02/msg00081.html
+
+- Fix case correctness of passwd/group entries fetched via getpwnam
+  or getgrnam.
+  Addresses: https://cygwin.com/ml/cygwin/2019-02/msg00109.html

winsup/cygwin/uinfo.cc

@@ -2046,6 +2046,16 @@ pwdgrp::fetch_account_from_windows (fetch_user_arg_t &arg, cyg_ldap *pldap)
       /* We can skip the backslash in the rest of this function. */
       if (p)
 	name = p + 1;
+      /* Reverse lookup name from sid to make sure the username in
+	 our passwd/group data is written exactly as in the user DB. */
+      nlen = UNLEN + 1;
+      dlen = DNLEN + 1;
+      ret = LookupAccountSidW (NULL, sid, name, &nlen, dom, &dlen, &acc_type);
+      if (!ret)
+	{
+	  system_printf ("LookupAccountNameW (%W), %E", name);
+	  return NULL;
+	}
       /* Last but not least, some validity checks on the name style. */
       if (!fq_name)
 	{