rgw: fix up the use of tenant before it's available

rgw: fix up the use of tenant before it's available

A big problem that cropped up with the multi-tenancy as it
went in was that it used tenant to initialize bucket names
before tenant was actually available by parsing a token.
Thus, although multi-tenancy worked, accessing anything across
tenants was actually impossible. This patch fixes that oversight.
Unfortunately, a new method postauth_init() was needed for it.

We also fix small bugs:

 - validate_tenant_name() used a proper-looking C++ but was
   incorrect. We simply use C for now, which may be not as
   pretty but is correct.

 - We now make a distinction between empty tenant syntax that
   means "the logged-in tenant" ("bucket") and the syntax
   that means "the legacy compatibility tenant with no name
   (":bucket").

 - We make rgw_make_bucket_entry_name() more robust against
   empty inputs. It's not used for work, but produces odd-looking
   debugging logs.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>