RXML: Fix parsing of <if expr> containing newlines.

Turns on the DOTALL mode (?s) in the rxml_var_splitter regexp.
Without this flag set, the rxml_var_splitter regexp fails on
multi-line expressions.

Thanks to Per Cederqvist <cederp@opera.com> for the report and fix.

Fixes [bug 7343]/[LysLysKOM 21003638].
Merge branch 'patches/bug7343' into 5.4

* patches/bug7343:
  RXML: Fix parsing of <if expr> containing newlines.