Also properly parse lsof to get second txt record instead of hoping the 5th line is the right one (wrong data returned for pid 57)

This code is based on the FreeBSD version
and implements roughly the same feature set.

system (no pgrep command) and one is a normal error state of pgrep
meaning no processes found for the criteria given (in this case the parent
pid does not exist or the process simply has no children).  The later case
is quite usefull to know about so I added a static error for this case.

Package common wasn't used for public functions. Place it in an
internal directory to prevent other packages from using.

Remove the distributed references to "HOST_PROC" and "HOST_SYS"
consts and combine into a common function. This also helps so that
if a env var is defined with a trailing slash all will continue to
work as expected.

Fixes #100