Fix two small bugs in tcsetattr().
Fix two small bugs in tcsetattr().

- According to POSIX, tcsetattr() must not fail when any of the bits in
  the structure are unsupported, but it must leave the unsupported flags
  alone.

- The CIGNORE flag (set by TCSASOFT, extension) was not cleared from
  c_cflag, which means using it would cause it to be applied during its
  entire lifespan. Eventually make sure we clear the flag.

I don't really like CIGNORE, but I think we must keep it alive right
now. With our new TTY layer, we don't actually need this mechanism,
because if you leave c_cflag, c_ispeed and c_ospeed alone, we won't make
a call into the device driver anyway.

Reported by:	naddy
Tested by:	naddy

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase