Only replace space with the specified substitution
    
    https://fedorahosted.org/sssd/ticket/2397
    
    - make sss_replace_whitespaces only replace space (' ') not any
      whitespace
    - make sss_replace_whitespaces only replace a single char, not the whole
      string
    - rename CONFDB_NSS_OVERRIDE_DEFAULT_WHITESPACE to
      CONFDB_NSS_OVERRIDE_DEFAULT_SPACE
    - rename the override_default_whitespace option to override_space
    - rename sss_replace_whitespaces() to sss_replace_space()
    - rename sss_reverse_replace_whitespaces() to sss_reverse_replace_space()
    - rename nctx->override_default_wsp_str to nctx->override_space
    - make the return value of sss_replace_space non-const to avoid freeing
      the result without compilation warnings
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>