points have the cygdrive prefix.
* (mount_info::add_reg_mount): Properly catch errors on registry operations.
* (mount_info::write_cygdrive_info_to_registry): Ditto.
* (mount_info::del_reg_mount): Cosmetic changes to be consistent with other
methods.
* (mount_info::add_item): Check arguments more precisely. Increment nmounts
only when registry operations succeed.