Cygwin: gethostname: fix fetching hostname from non-winsock function

If gethostname() fails we call GetComputerNameEx with
ComputerNameDnsFullyQualified.  This is wrong, gethostname should return
the hostname only, not the FQDN.  Fix this by calling GetComputerNameEx
with ComputerNameDnsHostname.

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

winsup/cygwin/net.cc

@@ -718,7 +718,7 @@ cygwin_gethostname (char *name, size_t len)
 	{
 	  DWORD local_len = len;
 
-	  if (!GetComputerNameExA (ComputerNameDnsFullyQualified, name,
+	  if (!GetComputerNameExA (ComputerNameDnsHostname, name,
 				   &local_len))
 	    {
 	      if (GetLastError () == ERROR_MORE_DATA)

winsup/cygwin/release/2.12.0

@@ -79,3 +79,5 @@ Bug Fixes
   Addresses: https://cygwin.com/ml/cygwin/2018-12/msg00173.html
 
 - Fix thread names in GDB when cygthreads get reused.
+
+- Fix return value of gethostname in a border case.