New dd conv= symbols nocreat, excl, fdatasync, fsync,

New dd conv= symbols nocreat, excl, fdatasync, fsync,
and new dd options iflag= and oflag=.

(usage): Document.
(fdatasync) [!HAVE_FDATASYNC]: New macro.
(C_NOCREAT, C_EXCL, C_FDATASYNC, C_FSYNC): New macros.
(input_flags, output_flags): New vars.
(LONGEST_SYMBOL): New macro.
(struct symbol_value): Renamed from struct conversion.  Members
symbol and value renamed from convname and conversion.  The
symbol value is now an array instead of a pointer; this saves
a bit of space and time in practice.  All uses changed.
(conversions): Add nocreat, excl, fdatasync, fsync.  Now const.
(flags): New constant array.
(iflag_error_msgid, oflag_error_msgid): New constants.
(parse_symbols): Renamed from parse_conversion and generalized
to handle either conversion or flag symbols.
(scanargs): Adjust uses of parse_symbols accodingly.  Add
support for iflag= and oflag=.  Reject attempts to use
both excl and nocreat.
(set_fd_flags): New function.
(dd_copy): Just return X rather than calling quit (X), since our
caller invokes quit with the returned value.  Add support for
fdatasync and fsync.
(main): Add support for iflags=, oflags=, and new conv= symbols.