modules/crypto/: move legacy modules to subdirectory

modules/crypto/: move legacy modules to subdirectory

This does not rename the modules themselves; you still load them with the
standard modules/crypto/ prefix. This is just to tidy up the source tree.

This also removes the modules/crypto/posix compatibility module. Please
see the Password Hashing Modules section of dist/atheme.conf.example if
you still use this module on v7.2 or older, for migration instructions.

Finally, document the ./configure options necessary to have each module
built, and elaborate on the distinction between pbkdf2 and pbkdf2v2.