cnitool: fix default CNI directory on Windows and add extra checks.

[aznashwan]

No description provided.

[aznashwan]

Note that the only new validation logic which may introduce extra constraints is this new filepath.IsAbs() check, but I can't think of a usecase where one might want to use a relative path instead of an absolute path.

I'm also open to any better suggestions for the Windows default path than this default from containerd.

[coveralls]

Coverage: 72.359%. Remained the same when pulling 289c64b on aznashwan:fix-default-windows-confdir into 6a43fb5 on containernetworking:main.

[aznashwan]

Note that the only new validation logic which may introduce extra constraints is this new filepath.IsAbs() check

FWIW I have switched to using filepath.Abs() to bring it in line with the rest of the code.

I've also compared the pre and post-patch behavior on Linux:

### Pre-patch:
# Relative paths work:
$ NETCONFPATH=../../../../etc/cni/net.d/ ./cnitool check nonexistent-net /var/run/netns/testing 
no net configuration with name "nonexistent-net" in ../../../../etc/cni/net.d/  # correct 

# Setting a nonexistent directory will only claim there are no net configurations:
$ NETCONFPATH=/nonexistent/path ./cnitool check nonexistent-net /var/run/netns/testing 
no net configurations found in /nonexistent/path

# Setting a nonexistent relative directory will also only claim there are no net configurations:
$ NETCONFPATH=./nonexistent/relative/path ./cnitool check nonexistent-net /var/run/netns/testing 
no net configurations found in ./nonexistent/relative/path

### Post-patch:
# Relative paths still work:
$ NETCONFPATH=../../../../etc/cni/net.d/ ./cnitool check nonexistent-net /var/run/netns/testing 
no net configuration with name "nonexistent-net" in /etc/cni/net.d  # correct

# Setting a nonexistent directory will error out with a clear message:
$ NETCONFPATH=/nonexistent/path ./cnitool check nonexistent-net /var/run/netns/testing 
the provided CNI config path ($NETCONFPATH) does not exist: "/nonexistent/path"

# Setting a nonexistent relative directory will also error out with a clear message:
$ NETCONFPATH=./nonexistent/relative/path ./cnitool check nonexistent-net /var/run/netns/testing 
the provided CNI config path ($NETCONFPATH) does not exist: "/home/nashu/cni/cnitool/nonexistent/relative/path"

[dcbw]

@aznashwan this seems containerd specific, but CNI is used by more than just containerd. If each CRI implementation has its own default directory, should this be a list of default dirs that get tried in sequence?