Fix FreeBSD readArguments and implement its readEnvironment

* FreeBSD's readArguments() implementation was broken:
    It read the wrong sysctl.
    Also the string processing did not work as expected.
* FreeBSD's readEnvironment() was not implemented.
    This provides an implementation using libprocstat's
     procstat_getenvv

Tested on my system w/ FreeBSD 11.0-CURRENT

Patch by Tobias Berner tcberner gmail com

Part of REVIEW 127525