illumos: misc portability fixes.

* Include termios.h in openpty.h. Under linux and darwin, ioctl.h
  provides `winsize`. Under illumos, it doesn't. For portability,
  explicitly include termios.h.
* Fix macro pollution in StackTrace.h. Under illumos, regset.h defines a
  set of unprefixed gregset_t index macros (ERR, FS, etc.). ClickHouse
  only uses the analogous prefixed macros (REG_ERR, REG_FS, etc.), so we
  conditionally undef the conflicting macros.
* Fix illumos-specific double/float cast errors in FPEnvironment, so
  that we don't widen or truncate inappropriately.
* Set `-z rescan-now` on illumos so that we don't have to manage
  dependencies in DEPENDS.

Part of #23777.