Add tests for the "tkey-gssapi-credential" option

Four named instances in the "nsupdate" system test have GSS-TSIG support
enabled.  All of them currently use "tkey-gssapi-keytab".  Configure two
of them with "tkey-gssapi-credential" to test that option.

As "tkey-gssapi-keytab" and "tkey-gssapi-credential" both provide the
same functionality, no test modifications are required.  The difference
between the two options is that the value of "tkey-gssapi-keytab" is an
explicit path to the keytab file to acquire credentials from, while the
value of "tkey-gssapi-credential" is the name of the principal whose
credentials should be used; those credentials are looked up in the
keytab file expected by the Kerberos library, i.e. /etc/krb5.keytab by
default.  The path to the default keytab file can be overridden using by
setting the KRB5_KTNAME environment variable.  Utilize that variable to
use existing keytab files with the "tkey-gssapi-credential" option.

The KRB5_KTNAME environment variable should not interfere with the
"tkey-gssapi-keytab" option.  Nevertheless, rename one of the keytab
files used with "tkey-gssapi-keytab" to something else than the contents
of the KRB5_KTNAME environment variable in order to make sure that both
"tkey-gssapi-keytab" and "tkey-gssapi-credential" are actually tested.

(cherry picked from commit 1746d2e8)
86 jobs for !4934 with 2634-test-tkey-gssapi-credential-v9_11 in 15 minutes and 24 seconds (queued for 9 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
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autoreconf

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clang-format

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coccinelle

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danger

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misc

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  Build
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clang:freebsd12:amd64

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clang:openbsd:amd64

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clang:tsan:noatomics

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gcc:alpine3.13:amd64

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gcc:asan

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gcc:buster:amd64

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gcc:centos6:amd64

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i386 linux
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gcc:centos7:amd64

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gcc:focal:amd64

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gcc:out-of-tree

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gcc:sid:amd64

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gcc:stretch:amd64

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gcc:tsan

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gcc:tsan:noatomics

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gcc:tumbleweed:amd64

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amd64 linux
gcc:xenial:amd64

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amd64 windows
msvc:windows:amd64

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unit:gcc:alpine3.13:amd64

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amd64 linux
unit:gcc:focal:amd64

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unit:gcc:pkcs11

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  System
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system:gcc:alpine3.13:amd64

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  Docs
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docs

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  Postcheck
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gcov

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