libmount: add support for verity devices via libcryptsetup

libmount: add support for verity devices via libcryptsetup

The following new options are added:

verity.hashdevice
verity.roothash
verity.hashoffset

The source path will be used as a dm-verity object, and will be
opened using libcryptsetup APIs.

A new --with-cryptsetup build-time option is added, which adds a
dependency on libcryptsetup. To ease bootstrapping, given libcryptsetup
build-depends on util-linux for libuuid, if --with-cryptsetup=yes but
libcryptsetup is not installed only a warning will be printed at
configure time rather than an error. This way stage0/first stage/ring0
builds can use the same configure options but avoid installing
cryptsetup to get a working base set, and then rebuild util-linux in
the next step of the boostrapping process.

If verity options are selected but cannot be fullfilled due to lack of
dependencies, mounting a volume will fail even if using a loop device
would work as a fallback, to avoid silently skipping integrity checks.