ITS#9203, ITS#9206

Patches from Peter Marschall (bugs have IPR notices):

• add a manual page for pw-argon2
• tell libsodium explicitly to use Argon2 hash
• treat memory param as KiB everywhere

Patches from me:

• s/slapd-argon2/pw-argon2/g README (based on patch from Peter)
• call sodium_init() before calling libsodium funcs
• convert crypto_pwhash_MEMLIMIT_INTERACTIVE to KiB
• use the argon2id default values explicitly
• don't document the (libargon2 specific) parameter defaults