Add userland stacks and threads commands

[pcd1193182]

Now that the stacks and threads commands have been properly separated into the kernel module, we can write a userland implementation of these commands. These implementations are heavily based on the originals, but using the generic drgn API that doesn't assume it can access the thread's object parameter. As a result, we have to lose the module and state filters on the stacks.

[codecov-commenter]

Codecov Report

Merging #321 (dbfdc2e) into 6.0/stage (f5e1943) will decrease coverage by 1.99%.
The diff coverage is 36.75%.

❗ Current head dbfdc2e differs from pull request most recent head b5127bb. Consider uploading reports for the commit b5127bb to get more accurate results

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@@              Coverage Diff              @@
##           6.0/stage     #321      +/-   ##
=============================================
- Coverage      87.81%   85.83%   -1.99%     
=============================================
  Files             64       66       +2     
  Lines           2742     2859     +117     
=============================================
+ Hits            2408     2454      +46     
- Misses           334      405      +71     

Impacted Files	Coverage Δ
sdb/commands/ustacks.py	31.13% <31.13%> (ø)
sdb/commands/uthreads.py	90.90% <90.90%> (ø)

... and 1 file with indirect coverage changes

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[ahrens]

can we add a test case for this?

[sdimitro]

@ahrens @pcd1193182

In terms of testing we can add a userland core dump to our test suite the same way we have kernel ones but the testing infrastructure will need to be adjusted a bit more. I can slowly start working on this. In the meantime I'm good with merging this.

EDIT: Paul if you have a core dump that you used for development, you can try archiving it with savedump and send me over the file.