(copy1): New function.
(copyenc): New function.
(mount_item::fnmunge): Return error condition. Use new functions to copy
strings.
(mount_item::build_win32): Ditto.
(mount_info::conv_to_win32_path): Return error condition.
(cygwin_conv_to_posix_path): Return result of path conversion.
(cygwin_conv_to_full_posix_path): Ditto.
(return_with_errno): New macro.
(cygwin_win32_to_posix_path_list): Use new macro to potentially set errno.
(cygwin_posix_to_win32_path_list): Ditto.
* path.h (mount_item::fnmunge): Add size argument.
(mount_item::build_win32): Ditto.
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