spec, libcni, pkg/invoke: Use OS-agnostic separator when parsing CNI_PATH

spec, libcni, pkg/invoke: Use OS-agnostic separator when parsing CNI_PATH

Hardcoding the list separator character as ":" causes CNI to fail when parsing
CNI_PATH on other operating systems. For example, Windows uses ";" as list
separator because ":" can legally appear in paths such as "C:\path\to\file".
This change replaces use of ":" with OS-agnostic APIs or os.PathListSeparator.

Fixes #358