Don't delete everything under a string-subkeyed primary key by using a

Don't delete everything under a string-subkeyed primary key by using a
loop that fetches the next item using conf_get_str_strs and passing
the previous key as a parameter, because the previous key will have
been freed by the intervening conf_del_str_str. Instead, use the
technique of repeatedly using conf_get_str_nthstrkey with index 0 and
deleting what comes back, as PSCP and PSFTP do.

Spotted by Minefield with the aid of Jacob, or possibly vice versa.

[originally from svn r9220]