Fix mosquitto_passwd hash types and some doc. details

[mikini]

• use WITH_ARGON2 define to only set argon2id as default and accept
  argon2id type if enabled (currently only possible on build cmd line)

• additionally use WITH_ARGON2 to produce corresponding inline help

• in doc. remove argon2id and indicate correct sha512-pbkdf2 default

• in doc. and inline help correct -c overwrite wording about effect of file
  existence and elaborate about expectations if not given

• in doc. and inline help document -I option for sha512-pbkdf2 iterations

(observations triggered by discussion in #2915)
(based on develop as instructed fixes branch does not carry changes since
~2.0, however develop is also missing changes since 2.1.0, ie. two tagged
releases, let me know if there is a better approach)

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