* external.cc (get_cygdrive_prefixes): New function.
(cygwin_internal): Add CW_GET_CYGDRIVE_PREFIXES case.
* path.cc (mount_info::read_cygdrive_info_from_registry): Read system cygdrive
prefix if user one is undefined.
(mount_info::write_cygdrive_info_to_registry): Write cygdrive prefix to the
appropriate registry hive. Overwrite in-memory copy of cygdrive, if
appropriate.
(mount_info::remove_cygdrive_info_from_registry): New method.
(mount_info::get_cygdrive_prefixes): New method.
(cygwin_umount): Remove cygdrive prefix, if appropriate.
* registry.cc (reg_key::killvalue): New method.
* shared.h (class reg_key): Add killvalue, remove_cygdrive_info_to_registry,
and get_cygdrive_prefixes declarations.
* include/sys/cygwin.h (cygwin_getinfo_types): Add CW_GET_CYGDRIVE_PREFIXES.
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