equivalent to not_executable.
* sys/mount.h: Define MOUNT_NOTEXEC.
* path.h (fs_info): New class.
(path_conv): Move fs-specific fields to new 'fs' structure.
(path_conv::update_fs_info): Move to fs_info and rename to just 'update'.
* path.cc (fs_info::update): Ditto. Return 'true' if successful.
(fillout_mntent): Add ',noexec' to list of reported options.