mitogen.parent.cfmakeraw() contains repeated flags

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E.g. BRKINT is specified twice

https://github.com/mitogen-hq/mitogen/blob/b8b15580af509830eb2b53fd86242345aaf3def7/mitogen/parent.py#L249-L256

Discovered during review of #1089

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>>> 'IMAXBEL IXOFF INPCK BRKINT PARMRK ' 'ISTRIP INLCR ICRNL IXON IGNPAR' ' ' 'IGNBRK BRKINT PARMRK'
'IMAXBEL IXOFF INPCK BRKINT PARMRK ISTRIP INLCR ICRNL IXON IGNPAR IGNBRK BRKINT PARMRK'
>>> sorted(_.split())
['BRKINT', 'BRKINT', 'ICRNL', 'IGNBRK', 'IGNPAR', 'IMAXBEL', 'INLCR', 'INPCK', 'ISTRIP', 'IXOFF', 'IXON', 'PARMRK', 'PARMRK']
>>> 'IMAXBEL IXOFF INPCK BRKINT PARMRK ' 'ISTRIP INLCR ICRNL IXON IGNPAR' ' ' 'IGNBRK BRKINT PARMRK'
'IMAXBEL IXOFF INPCK BRKINT PARMRK ISTRIP INLCR ICRNL IXON IGNPAR IGNBRK BRKINT PARMRK'
>>> sorted(_.split())
['BRKINT', 'BRKINT', 'ICRNL', 'IGNBRK', 'IGNPAR', 'IMAXBEL', 'INLCR', 'INPCK', 'ISTRIP', 'IXOFF', 'IXON', 'PARMRK', 'PARMRK']
>>> sorted(set(_))
['BRKINT', 'ICRNL', 'IGNBRK', 'IGNPAR', 'IMAXBEL', 'INLCR', 'INPCK', 'ISTRIP', 'IXOFF', 'IXON', 'PARMRK']
>>> ' '.join(_)
'BRKINT ICRNL IGNBRK IGNPAR IMAXBEL INLCR INPCK ISTRIP IXOFF IXON PARMRK'

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https://github.com/mitogen-hq/mitogen/wiki/cfmakeraw-notes

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Mitogen attempts to (un)set a few non-POSIX flags

• iflag
  • IMAXBEL=0 - Ring bell when input queue is full. Not implemented on Linux, always behaves as on.
• oflag
  • CREAD=1 - Enable receiver
• lflag
  • PENDIN=0 - Reprint characters in input queue when next source character is read. Not supported on Linux, marked XXX in https://man.freebsd.org/cgi/man.cgi?query=termios&sektion=4&apropos=0&manpath=FreeBSD+14.2-RELEASE+and+Ports#Local+Modes

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Mitogen does not modify a few flags that others do modify

• iflag
  • IGNCR - Ignore Carriage Return. Removed in https://github.com/mitogen-hq/mitogen/commit/56943d3141c95a25b376d4dcfe01741d22f78bdf
• cc
  • VMIN & VTIME - determine blocking beahviour of read() calls

Mitogen may silently (un)set any flag that has no corresponding entry in the Python module termios, by way of getattr(..., termios, 0).

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Previous discussion https://github.com/mitogen-hq/mitogen/issues/71