"override" entries.
(from_fstab_line): Handle mount points same as Cygwin itself. Use
"cygdrive prefix" as posix name of cygdrive prefix.
(from_fstab): Add auto entries for /usr/bin and /usr/lib. Add
MOUNT_AUTOMATIC and MOUNT_IMMUTABLE flags to default root dir entry.
Use "cygdrive prefix" as posix name of default cygdrive prefix.
(getmntent): Allocate smaller buffers. Align output closer to output
of mount(1).