mmnormalize (variable, allow_regex, title disambiguation) + rscript (foreach, reset, control-structures) + misc

Summary:
- title disambiguation (between mmnormalize and mmjsonparse)
- added docs for mmnormalize allow_regex (module param) and variable (action param)
- dedicated page for rscript control-structures
- detailed docs around behavior of 'set', 'unset' and 'reset'

This patch-set goes after: https://github.com/rsyslog/rsyslog/pull/149 (which in turn, goes after: https://github.com/rsyslog/liblognorm/pull/9)