rule-syntax-check: add support for escaped double quotes

rule-syntax-check: add support for escaped double quotes

Add support to backslash-escaped double quote inside a string.

Tested by modifying src/login/70-uaccess.rules to include:

  ACTION=="remove" it", GOTO="uaccess_end"

And had the rule checker complain about it:

  $ test/rule-syntax-check.py src/login/70-uaccess.rules
  # looking at src/login/70-uaccess.rules
  Invalid line src/login/70-uaccess.rules:10: ACTION=="remove" it", GOTO="uaccess_end"
    clause: ACTION=="remove" it"