gssapi: Fix support of delegated credentials
gssapi: Fix support of delegated credentials

In a previous refactoring patch, the code underpinning the
ssh_gssapi_set_creds() API was inadvertently removed. This patch
fixes the problem.

Also clarify what variable holds which credentials and insure that
credentials created within the library are propelry freed.

Signed-off-by: Simo Sorce <65f99581a93cf30dafc32b5c178edc6b0294a07f@redhat.com>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

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