fix fopen() overwriting stdin file handle

See adtools#2
Fix is extracted from the fork by @bebbo, specifically the two following commits:
bebbo/libnix@fa82b7a
bebbo/libnix@95dbee7

Author: sezero

Diff for: sources/nix/stdio/fopen.c

@@ -19,6 +19,7 @@ FILE *fopen(const char *filename,const char *mode)
   { if((node->FILE.buffer=(char *)malloc(BUFSIZ))!=NULL)
     { node->FILE.bufsize=BUFSIZ;
       node->FILE.flags|=__SMBF; /* Buffer is malloc'ed */
+      node->FILE.file = -1;
       if(freopen(filename,mode,&node->FILE)!=NULL)
       { AddHead((struct List *)&__filelist,(struct Node *)&node->node);
         return &node->FILE; }

Diff for: sources/nix/stdio/freopen.c

@@ -11,7 +11,8 @@ extern void __seterrno(void);
 FILE *freopen(const char *filename,const char *mode,FILE *stream)
 { int error=__fflush(stream);
 
-  close(stream->file);
+  if (stream->file >= 0)
+    close(stream->file);
   if(stream->name!=NULL) /* file is temporary */
   { BPTR cd=CurrentDir(stream->tmpdir); /* cd t: */
     if(!DeleteFile(stream->name))  /* delete file */

Diff for: sources/nix/stdio/open.c

@@ -40,7 +40,7 @@ static __inline StdFileDes *_allocfd(void)
   int file,max;
 
   for(sfd=stdfiledes,max=stdfilesize,file=0;file<max;sfd++,file++)
-    if(!sfd[0] || !sfd[0]->lx_inuse)
+    if(!sfd[0] || (!sfd[0]->lx_inuse && !sfd[0]->lx_sys))
       break;
 
   if(file>SHRT_MAX)