libmisc/salt.c: Sanitize code.

libmisc/salt.c: Sanitize code.

* Move all pre-processor defines to the top of the file.
* Unify the gensalt() function to be useable for all supported
  hash methods.
* Drop the gensalt_{b,yes}crypt() functions in favor of the
  previous change.
* Refactor the functions converting the rounds number into
  a string for use with the crypt() function, to not require
  any static buffer anymore.
* Clarify the comment about how crypt_make_salt() chooses the used
  hash method from the settings in the login.defs file.
* Use memset() to fill static buffers with zero before using them.
* Use a fixed amount of 16 random base64-chars for the
  sha{256,512}crypt hash methods, which is effectively still less
  than the recommendation from NIST (>= 128 bits), but the maximum
  those methods can effectively use (approx. 90 bits).
* Rename ROUNDS_{MIN,MAX} to SHA_ROUNDS_{MIN,MAX}.
* Bugfixes in the logic of setting rounds in BCRYPT_salt_rounds().
* Likewise for YESCRYPT_salt_cost().
* Fix formatting and white-space errors.

Signed-off-by: Björn Esser <besser82@fedoraproject.org>