Bug#932000: libgssapi-krb5-2: gss_krb5int_set_allowable_enctypes breaks NFSv4 after removal of deprecated DES enctypes in 1.17-4

[Greg Hudson]

I think gss_krb5int_set_allowable_enctypes() should filter out invalid
enctypes, and only fail if no enctypes remain after filtering.  The
current logic would also fail if the kernel supported an enctype which
libkrb5 did not (e.g. if the kernel gained support for the aes-sha2
enctypes, and someone upgraded to a new kernel on a system with
krb5-1.14 or previous).