[core] adjust parser for valid variable expansion
[core] adjust parser for valid variable expansion

adjust config parser for valid variable expansion
Return only the value when a variable is expanded so that the
array element keeps its state as value-only or part of key-value

(thx nicorac)

x-ref:
  "https://redmine.lighttpd.net/boards/2/topics/7600"

lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4