Factor code into two new functions in preparation for a future commit.

Factor code into two functions.
read_exportfile() a functon which reads the exports file(s) and calls
get_exportlist_one() to process each of them.
delete_export() a function which deletes the exports in the kernel for a file
system.
The contents of these functions is just the same code as was used to do the
operations, moved into separate functions. As such, there is no semantic change.
This is being done in preparation for a future commit that will add an
option to do incremental changes of kernel exports upon receiving SIGHUP.

MFC after: 1 month