ITS#6035 saslauthz cleanups (no functional change)

- give authid-rewrite's argument a name
- tidy saslauthz.c whitespace (mixed spaces/tabs)
- always declare slap_sasl_regexp_destroy: fixes an implicit declaration
  warning when configured without librewrite
- delete dead code: ENABLE_REWRITE implies SLAP_AUTH_REWRITE, so this
  code is never compiled
- make slap_sasl_regexp_rewrite_config static
- omit sasl_regexp unused fields when built with librewrite