files.cf: set_config_values changed to support special RHS chars and smarter operation

Changed set_config_values to support special characters in the RHS (so we don't confuse the regex on expansion) and:
1. if LHS exists with the wrong value correct it, else
2. if LHS is commented out and #1 did not apply, uncomment and correct, elst
3. if #1 and #2 did not apply, append it

Plus other fixes.  See https://github.com/cfengine/masterfiles/pull/4
lib/3.6/files.cf: add set_config_values acceptance test
Merge pull request #53 from tzz/ed45626/feature/improve-file-editing

files.cf: set_config_values changed to support special RHS chars and sma...