terminal handling fixes for Neovim 0.10

[jclulow]

A few fixes for :terminal support in Neovim 0.10:

• The openpty(3C) compatibility shim is not quite right in one critical respect: pushing a STREAMS module onto a tty device can have the side effect of making that device the process's controlling terminal. Using __I_PUSH_NOCTTY avoids that side effect. This was causing the nvim --embed child process to exit unexpectedly when :terminal was used.

• Neovim presently does some partially incorrect and wholly ill-advised construction of a struct termios object from whole cloth when starting an interactive child process. This likely has a subtle impact even on Linux (I verified that it is not quite producing a default object there either, though the differences are less severe). The delta between our system default termios object and the one Neovim produces is:

  -===== DEFAULT:
  +===== FROM NEOVIM:
  
  input flags (c_iflag):
  -       BRKINT   Signal interrupt on break
         ICRNL   Map CR to NL on input
          IXON   Enable start/stop output control
  -      IMAXBEL   Echo BEL on input line too long
  
  output flags (c_oflag):
         OPOST   Post-process output
         ONLCR   Map NL to CR-NL on output
         NLDLY:  Select newline delays
                   NL0
         CRDLY:  Select carriage-return delays
                   CR0
        TABDLY:  Select horizontal tab delays or tab expansion
  -                  TAB3
  -                  XTABS
  +                  TAB0
         BSDLY:  Select backspace delays
                   BS0
         VTDLY:  Select vertical tab delays
                   VT0
         FFDLY:  Select form feed delays
                   FF0
  
  control flags (c_cflag):
         CREAD   Enable receiver
  -      CRTSCTS   Enable outbound hardware flow control
      CBAUDEXT   Bit to indicate output speed > B38400
  +    CIBAUDEXT   Bit to indicate input speed > B38400
         CBAUD:  Baud rate (with CEXTBAUD)
  -                  B115200
  +                  B57600
         CSIZE:  Character size
                   CS8
  
  local flags (c_lflag):
  -       ICANON   Canonical input (erase/kill proc.)
        TOSTOP   Send SIGTTOU for background output
       ECHOPRT   Echo erase char as char erased
  -       FLUSHO   Output is being flushed
  
  control characters (c_cc):
     for canonical mode:
            VINTR     0x03 (  3)  ^C
            VQUIT     0x1c ( 28)
           VERASE     0x7f (127)
            VKILL     0x15 ( 21)  ^U
  -            VEOF     0x04 (  4)  ^D
  +            VEOF     0x01 (  1)  ^A
             VEOL     0x00 (  0)
            VEOL2     0x00 (  0)
           VSWTCH     0x00 (  0)
           VSTART     0x11 ( 17)  ^Q
            VSTOP     0x13 ( 19)  ^S
            VSUSP     0x1a ( 26)  ^Z
  -          VDSUSP     0x19 ( 25)  ^Y
  +          VDSUSP     0x00 (  0)
         VREPRINT     0x12 ( 18)  ^R
  -        VDISCARD     0x0f ( 15)  ^O
  +        VDISCARD     0x00 (  0)
          VWERASE     0x17 ( 23)  ^W
           VLNEXT     0x16 ( 22)  ^V
  -         VSTATUS     0x14 ( 20)  ^T
  -         VERASE2     0x08 (  8)  ^H
  +         VSTATUS     0x00 (  0)
  +         VERASE2     0x00 (  0)
     for raw mode:
  -            VMIN     0x04 (  4)  ^D
  +            VMIN     0x01 (  1)  ^A
            VTIME     0x00 (  0)

  Of perhaps paramount importance is that the terminal is in raw mode, which is generally totally unexpected on a UNIX system, and followed closely behind that is an unusual value for VEOF, which happens to share a slot with VMIN which has been spuriously overridden. I have removed the overrides entirely so that we just use the system default settings. I'll be working up the energy to post that fix in particular upstream, as I do not believe it is ever correct to do what they're doing there.

With these fixes in place, an interactive :terminal works under Neovim, as does the fzf-lua plugin that is quite popular and which depends on running fzf in under a pty.