When adding a column or index that uses plugin-defined
sysvar-based options with CREATE ... LIKE the server
was using the current value of the sysvar, not the default one.
Because parse_option_list() function was used both in create
and open and it tried to guess when it's create (need to use
current sysvar value and add a new name=value pair to the list)
or open (need to use default, without extending the list).
Let's move the list extending functionality into a separate
function and call it explicitly when needed. Operations that
add new objects (CREATE, ALTER ... ADD) will extend the list,
other operations (ALTER, CREATE ... LIKE, open) will not.
