util-lib: rework get_process_cmdline() (#3529)
This reworks get_process_cmdline() quite substantially, fixing the following:
- Fixes:
https://github.com/elogind/elogind/pull/3512/commits/
a4e3bf4d7ac2de51191ce136ee9361ba319e106c#r66837630
- The passed max_length is also applied to the "comm" name, if comm_fallback is
set.
- The right thing happens if max_length == 1 is specified
- when the cmdline "foobar" is abbreviated to 6 characters the result is not
"foobar" instead of "foo...".
- trailing whitespace are removed before the ... suffix is appended. The 7
character abbreviation of "foo barz" is hence "foo..." instead of "foo ...".
- leading whitespace are suppressed from the cmdline
- a comprehensive test case is added