Bug fix

• python filters would cause "invalid escape sequence" SyntaxWarnings
  with python 3.12 or newer.

• --skip-ctty option skips making slave pty a controlling terminal

• warn (don't die) when a specified completion file is not found

• configure will bail out (not just warn) when no termcap -like
  library is found.

• additionally check for TERM when INSIDE_EMACS is set
  (allowing the use of rlwrap within shells run by emacs)

• gcc -Wformat-security would warn about rl_message() being
  used in an unsafe manner.

• silence unhelpful input/output error messages at command exit