shared: split out crypt() specific helpers into its own .c/.h in src/shared/

shared: split out crypt() specific helpers into its own .c/.h in src/shared/

This way we can use libxcrypt specific functionality such as
crypt_gensalt() and thus take benefit of the newer algorithms libxcrypt
implements. (Also adds support for a new env var $SYSTEMD_CRYPT_PREFIX
which may be used to select the hash algorithm to use for libxcrypt.)

Also, let's move the weird crypt.h inclusion into libcrypt.h so that
there's a single place for it.