Add support for chdir and getcwd

Add chdir and getcwd and store cwd in the nfs_context.
Add functions to process the paths specified and normalize them
by performing the transforms :
    // -> /
    /./ -> /
    ^/../ -> error
    ^[^/] -> error
    /string/../ -> /
    /$ -> \0
    /.$ -> \0
    ^/..$ -> error
    /string/..$ -> /

Update the path lookup function to allow specifying relative paths based on
cwd for all functions.