cryptsetup: add option to use via dlopen in libmount

cryptsetup: add option to use via dlopen in libmount

Enabling libcrypsetup in libmount had several unintended side
effects.
First of all, it increases the Debian minimal image size by
~2.5% (5.6MB worth of new libraries).
Then, due to libcryptsetup linkage to OpenSSL and libjson-c,
it causes incompatibilities with external programs linking
against both libmount and a private, static, old version of
OpenSSL, or external programs linking against libjansson or
json-glib, which have one symbol in common with libjson-c.

If ./configure is ran with --with-crypsetup=dlopen,
instead of linking to libcrypsetup, use dlopen to resolve
the symbols at runtime only when the verity feature is
used, thus avoiding clashes and keeping images size down.

Fixes #1081

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>