* set_targ.adb (Read_Target_Dependent_Values):
close target description file once its contents is read.
* s-os_lib.adb (Non_Blocking_Spawn, version with Stdout_File
and Stderr_File): Close local file descriptors before spawning
child process.
* exp_util.adb (Containing_Package_With_Ext_Axioms): Limit scope of
local variables to make the code easier to understand and avoid
duplicated calls to Parent and Generic_Parent.