shared/sysctl-util: normalize repeated slashes or dots to a single value

We use those strings as hash keys. While writing "a...b" looks strange,
"a///b" does not look so strange. Both syntaxes would actually result in the
value being correctly written to the file, but they would confuse our
de-deplication over keys. So let's normalize. Output also becomes nicer.

Add test.

(cherry picked from commit f3b136a4)