New mount API: implement fsopen(2), fsconfig(2), fsmount(2), move_mount(2).

[copybara-service]

New mount API: implement fsopen(2), fsconfig(2), fsmount(2), move_mount(2).

The first step in implementing the new file-descriptor based mount API in
gVisor. A "filesystem creation context" is created using fsopen(). The context
is then configured using fsconfig(), and the FSCONFIG_CMD_CREATE command is
used to create a filesystem. A "mount object" can be obtained using fsmount(),
which can be operated on directly using the *at() functions or mounted onto the
real directory tree using move_mount().

For compatibility reasons, the new mount API is gated behind the runtime flag
--new-mount-api, which is disabled by default.

The current implementation has the following limitations:

• Linux passes configuration options to filesystems on-the-fly as fsopen() and
  fsconfig() are called, allowing per-parameter error handling. gVisor
  currently queues up all options and passes them to the filesystem all-at-once
  when FSCONFIG_CMD_CREATE occurs. Lifting this will require refactoring the
  option parsing for all existing filesystems.
• Only flag and string arguments to fsconfig() are supported.
• Reconfiguration of the filesystem through fsconfig() is not supported.
• move_mount(): flags MOVE_MOUNT_SET_GROUP and MOVE_MOUNT_BENEATH are not
  supported.
• The filesystem context message retrieval interface is not supported.

In the interest of review size, there is no implementation yet for
open_tree(2) or fspick(2).