Part 1 of file descriptor race and deadlock corrections.

File status flags should be shared for dup'ed file descriptors.
However fd_table_entry's should not be shared for dup'ed file
descriptors so they can be independently be closed without
interfering with dup'ed fd's.

- split out file status flags into its own structure
fs_flags to manage sharing of status flags between
dup'ed file descriptors.
- when duplicating a fd, initialize a new fd_table_entry
for the new fd, but share the status flags via status_flags.
- consolidate the code that sets the underlying system fd
to be non-blocking to a new function _thread_fs_flags_init()
- consolidate the code that sets the underlying system
fd back to blocking into a new function _thread_fs_flags_replace()

This change is needed as a prerequisite to the coming race
and deadlock corrections. okay marc@

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