* spawn.cc (av::dup_maybe): Make function void rather than void *.

* environ.cc (environ_init): Remember to reparse CYGWIN if envp is
supplied.
* heap.cc (_sbrk): Remember frame for signal handling.
* syscalls.cc (read_handler): Eliminate.
(_read): Move read_handler code here.  Reorganize for one path through
'ready_for_read'.

winsup/cygwin/heap.cc

@@ -86,6 +86,7 @@ heap_init ()
 extern "C" void *
 _sbrk(int n)
 {
+  sigframe thisframe (mainthread);
   char *newtop, *newbrk;
   unsigned commitbytes, newbrksize;