Use gss_acquire_cred() to obtain credentials from the default
credentials store for client-based GENSEC GSSAPI operation.

This allows proper execution under GSSAPI interposer plugin provided by
GSS-proxy project. GSS-proxy 0.6.2 makes possible to interpose
gss_acquire_cred() in a similar fashion to gss_acquire_cred_from().
While gss_acquire_cred_from() is a MIT extension, gss_acquire_cred() is
available in Heimdal as well and is used in other parts of Samba, namely
in source3's GENSEC gse module.

Client operation in GSS-proxy-enabled environment is needed for upcoming
FreeIPA 4.5 which internaly does privilege separation between its
framework and Apache front end code that handles actual authentication
of incoming client requests.

Samba Python bindings are used by FreeIPA framework to establish forest
trust between local Samba and remote Active Directory forests. When
FreeIPA administrator attempts to create a trust, FreeIPA uses S4U2Proxy
to allow HTTP/... principal to act on behalf of the remote administrator
credentials. With privilege separation in place, Samba GENSEC and
credentials code is unable to find actual tickets in the credential
cache available in the environment because GSS-proxy keeps them
encrypted and only accessible via GSSAPI calls that were interposed.

gss_krb5_import_cred() is not interposed, as many other direct Kerberos
extensions to GSSAPI. As result, we need to use a proper GSSAPI call here.