Update zloop.sh to use coredumpctl

[behlendorf]

Motivation and Context

We'd like to be able to run zloop.sh and debug any generated core
files without requiring escalated privileges. The only thing preventing
this is the test scripts need to set the system wide core_pattern.

This change resolves that issue by instead using the coredumpctl
command to extract the dump and write it to the specific name core.
Once the core has been written, and same gdb script is used to
dump the most useful information.

One upshot of this is it makes it possible to start running zloop.sh
on the non-x86 CI builders which are not virtual and intentionally
do not allow passwordless sudo.

Description

On modern Linux systems there is no need to set a specific
/proc/sys/kernel/core_pattern, the coredumpctl dump command
can be used to extract the core using the specific name.

By removing the need to set the core_pattern zloop.sh can now
be run with normal user priviledges. Previous work has already
been done to allow ztest to run without any of the kmods being
loaded.

Additionally, cleanup a few remaining shellcheck warnings in
the zloop.sh script. Allow options to be passed as environment
variables. Print a message when running for a fixed duration.

Note: coredumpctl is not portable outside of Linux, but neither
is the rest of the script so at some point it will need to be updated
to do the right thing for each supported platform.

How Has This Been Tested?

Locally using Fedora 31 as a normal user with no additional
privileges. I verified coredumpctl is available in distributions
as old as CentOS 7 so it should work there as well, but that
has not yet been manually test yet.

To test start ./scripts/zloop.sh in a terminal and let it get going.
In a different terminal kill the ztest process and verify that
coredumpctl extracts the dump and the gdb script extracts
the required information from the code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9613 into master will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #9613    +/-   ##
========================================
+ Coverage      79%      79%   +<1%     
========================================
  Files         418      418            
  Lines      123577   123572     -5     
========================================
+ Hits        97951    98007    +56     
+ Misses      25626    25565    -61

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <ø> (ø)	⬇️

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[behlendorf]

Closing for now.