Merge:
* libzstd-dev from ticket32242_035
* shellcheck from maint-0.4.0

Merge
* Chutney Trusty deletion in bug32240_029
* NSS addition in maint-0.3.5

Closes 32240.

We need to set "Sandbox 0", until we fix sandbox errors that are
triggered by Ubuntu Xenial and Bionic. See 32722.

Part of 32240.

"ours" to avoid version bump.

"ours" to avoid version bump.

NSS:
* test NSS-specific code with -std=gnu99
* use a recent gcc version from the latest Ubuntu image

Chutney:
* use clang, so we keep one clang Linux job
* keep clang on a fast job, so the overall build finishes quickly

Closes ticket 32500 for 0.3.5.

Merge C_DIALECT_OPTIONS from ticket32500_029 with
other configure options from maint-0.3.5.

Require C99 standards-conforming code in Travis CI, but allow GNU gcc
extensions. Also activates clang's -Wtypedef-redefinition warnings.

Builds some jobs with -std=gnu99, and some jobs without.

Closes ticket 32500.

This time, we're disabling it because it's the lowest priority job.
(And also slow.)

Part of 32629.

Merge re-enable chutney from ticket32629_029 with
stem from maint-0.3.5.

This reverts commit 4c4a0d50.

This reverts commit 511aeba8.