Fix issue #4321 (backspace not working at conflict prompt).
Implement simple line editing on Unix when termios is available.

* subversion/libsvn_subr/prompt.c: Include svn_ctype.h and signal.h.
  (terminal_open): With termios, always turn off terminal echo.
  (echo_control_char): New; Helper function to echo control characters.
  (terminal_getc): In termios mode:
   - echo input characters, or asterisks when echo is disabled;
   - handle deletions and job control;
   - echo actual control chars we handled, even if they've been redefined.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449442 13f79535-47bb-0310-9956-ffa450edef68