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
This takes some of the machinery from CNI and from the rkt networking
code, and turns it into a library that can be linked into go apps.
Included is an example command-line application that uses the library,
called `cnitool`.
Other headline changes:
* Plugin exec'ing is factored out
The motivation here is to factor out the protocol for invoking
plugins. To that end, a generalisation of the code from api.go and
pkg/plugin/ipam.go goes into pkg/invoke/exec.go.
* Move argument-handling and conf-loading into public API
The fact that the arguments get turned into an environment for the
plugin is incidental to the API; so, provide a way of supplying them
as a struct or saying "just use the same arguments as I got" (the
latter is for IPAM plugins).
