rename '_poserror' to '__poserror'

The old name could be non-conforming to the C standard. The new name
is definitely in the "implementation name space".

See issue #1796.

libsrc/common/_poserror.c

@@ -1,6 +1,6 @@
 /*****************************************************************************/
 /*                                                                           */
-/*                                _poserror.c                                */
+/*                               __poserror.c                                */
 /*                                                                           */
 /*                   Output a system dependent error code                    */
 /*                                                                           */
@@ -39,7 +39,7 @@
 
 
 
-void __fastcall__ _poserror (const char* msg)
+void __fastcall__ __poserror (const char* msg)
 {
     /* Fetch the message that corresponds to _oserror */
     const char* errormsg = _stroserror (_oserror);