These funcs are only used in net_unix.go which is only compiled for
darwin and freebsd.

Signed-off-by: NKamil Dziedzic <arvenil@klecza.pl>

The order of init function execution is dependant on the order that the
source files are passed to the compiler. This causes issues when
building under other build systems, such as bazel or buck, as they are
not guarenteed to maintain the same file order as the default go tool.

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

This may wokrs all of the platform.