Allows to upgrade the cache using the sssctl tool, which might be useful
e.g. in RPM %post scripts.

Since we want to use the sssctl tool during upgrade, we need to amend
the tools initialization code to not error out if sysdb can't be
instantiated, but rather return errno and let the tool handle the error.

Each tool command now has a 'allowed errno' the command is able to
handle. In this patch iteration, only a single errno can be handled and
only the upgrade command is able to do so.

The function sysdb_init() is never used to allow upgrade, so the
allow_upgrade parameter was pointless.

For users and groups, convert the input name to the qualified format.

Resolves:
    https://fedorahosted.org/sssd/ticket/3059

Use sss_create_internal_fqname for internal cache lookups. Because the
object's existence is verified using getpw* and getgr*, we keep using
sss_tc_fqname there, just to feed the NSS interface the expected
qualified or unqualified name format.

Same as all other tools.

All users and groups are now stored in the cache using the same format,
so we can use that one instead of creating a domain-specific name.

No need to export functions that are only used internally.

Normally we convert the names from short to internal format on input.
For the local domain tools, we can consider the sss_sync_ops an input
interface, to avoid having to convert the name in each tool and
interface separately.

Resolves:
https://fedorahosted.org/sssd/ticket/2247

Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>

Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>