Use longjmp at setjmp where possible

Where we can we should use longjmp and setjmp in preference to swapcontext/
setcontext as they seem to be more performant.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_posix.c

@@ -75,6 +75,7 @@ int ASYNC_FIBRE_init(ASYNC_FIBRE *fibre)
     fibre->fibre.uc_stack.ss_sp = stack;
     fibre->fibre.uc_stack.ss_size = STACKSIZE;
     fibre->fibre.uc_link = NULL;
+    fibre->env_init = 0;
 
     return 1;
 }