Enforce the recent flags cleanups - we want to use 'unsigned int flags'
in any of our APIs (except where backwards compatibility is important,
in the public migration APIs), and that all flags are checked for
validity (except when there are stub functions that completely
ignore the flags argument).

There are a few minor tweaks done here to avoid false positives:
signed arguments passed to open() are renamed oflags, and flags
arguments that are legitimately ignored are renamed flags_unused.

* cfg.mk (sc_flags_usage): New rule.
(exclude_file_name_regexp--sc_flags_usage): And a few exemptions.
(sc_flags_debug): Tweak wording.
* src/util/iohelper.c (runIO, main): Rename variable.
* src/util/util.c (virSetInherit): Likewise.
* src/fdstream.h (virFDStreamOpenFile, virFDStreamCreateFile):
Likewise.
* src/fdstream.c (virFDStreamOpenFileInternal)
(virFDStreamOpenFile, virFDStreamCreateFile): Likewise.
* src/util/command.c (virExecWithHook) [WIN32]: Likewise.
* src/util/util.c (virFileOpenAs, virDirCreate) [WIN32]: Likewise.
* src/locking/lock_manager.c (virLockManagerPluginNew)
[!HAVE_DLFCN_H]: Likewise.
* src/locking/lock_driver_nop.c (virLockManagerNopNew)
(virLockManagerNopAddResource, virLockManagerNopAcquire)
(virLockManagerNopRelease, virLockManagerNopInquire): Likewise.

This is needed if we want to transfer a temporary file. If the
transfer is done with iohelper, we might run into a race condition,
where we unlink() file before iohelper is executed.

* src/fdstream.c, src/fdstream.h,
  src/util/iohelper.c: Add new option
* src/lxc/lxc_driver.c, src/qemu/qemu_driver.c,
  src/storage/storage_driver.c, src/uml/uml_driver.c,
  src/xen/xen_driver.c: Expand existing function calls

The O_NONBLOCK flag doesn't work as desired on plain files
or block devices. Introduce an I/O helper program that does
the blocking I/O operations, communicating over a pipe that
can support O_NONBLOCK

* src/fdstream.c, src/fdstream.h: Add non-blocking I/O
  on plain files/block devices
* src/Makefile.am, src/util/iohelper.c: I/O helper program
* src/qemu/qemu_driver.c, src/lxc/lxc_driver.c,
  src/uml/uml_driver.c, src/xen/xen_driver.c: Update for
  streams API change