Add a new 'rc-manager' configuration parameter that allows to select
the strategy used to write resolv.conf; currently supported values
are: none|resolvconf|netconfig, 'none' meaning that NM directly writes
the file.

The default value of the parameter is 'none'; however if a
RESOLVCONF_PATH (or NETCONFIG_PATH) is specified at build time, the
default value will be 'resolvconf' (or 'netconfig').