Allow users to omit specific options from dsm.{opt,sys}.

This allows users of the tsmclient definition to specify a value of an
option (in the 'optoptions' and 'sysoptions' parameters) as `false'.
Doing so will cause that option to be omitted from dsm.opt/dsm.sys,
even if it has a value in tsmclient::config::default_{opt,sys}options.