Fix clean-unix rule for gssapi_krb5.h

Don't delete gssapi_krb5.h now that it is not generated.

[ghudson@mit.edu: also remove from generate-files-mac rule and from
.gitignore]
Don't leak the creds we don't plan to copy over
Check for not having found an entry properly

Return KRB5_KDB_NOENTRY if we failed to set a return entry, not if the
destination pointer for it is still NULL after we initialize it.
Handlers won't take care of initializing 'err'

We can get an KRB5_KDC_UNREACH error back without processing a server
reply first, so we should go ahead and initialize err for that case.
Don't leak the per-request preauth context