Fill in dt_pthread_setname() support for BSD's.

[brad0]

Summary

Fill in dt_pthread_setname() support for BSD's.

Use the appropriate _np pthread setname functions for FreeBSD,
NetBSD, OpenBSD and DragonFly.

Checklist

• I have read CONTRIBUTING.md and the coding style.
• I have not merged master into the topic branch.
• The pull request is one logical change, and every commit compiles on its own.
• I ran the relevant tests: unit tests, src/tests/integration/ where the pixelpipe is touched, or darktable-cli as a headless smoke test.
• New user-visible strings use _(), new preferences are registered in data/darktableconfig.xml.in.
• A RELEASE_NOTES.md entry was added (only needed if fixing an issue in a release). Do not reference GitHub issues.

Test instructions

On OpenBSD, with top. Use H to toggle threads.

  PID      TID PRI NICE  SIZE   RES STATE     WAIT      TIME    CPU COMMAND
56319   227806   2    0  116M  116M sleep/1   kqread    0:05  4.30% darktable
56319   418507  10    0  116M  116M idle      fsleep    0:00  0.00% darktable/[pango] fontcon
56319   403825  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   392820  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   246946  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   200559  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   413285  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   348894  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   478712  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   257836  10    0  116M  116M idle      fsleep    0:00  0.00% darktable
56319   574022  10    0  116M  116M sleep/1   nanoslp   0:00  0.00% darktable/gphoto_update
56319   293833   2    0  116M  116M idle      kqread    0:00  0.00% darktable/lua thread
56319   211062  10    0  116M  116M sleep/1   fsleep    0:00  0.00% darktable/worker 2
56319   322377   2    0  116M  116M idle      kqread    0:00  0.00% darktable/gdbus
56319   241846   2    0  116M  116M sleep/2   kqread    0:00  0.00% darktable
56319   294194   2    0  116M  116M idle      kqread    0:00  0.00% darktable/gmain
56319   183049  10    0  116M  116M idle      fsleep    0:00  0.00% darktable/pool-spawner
56319   175674  10    0  116M  116M sleep/4   nanoslp   0:00  0.00% darktable/worker 0
56319   151262  10    0  116M  116M sleep/2   fsleep    0:00  0.00% darktable/worker 1
56319   346194  10    0  116M  116M sleep/2   nanoslp   0:00  0.00% darktable/kicker
56319   158050  10    0  116M  116M sleep/2   fsleep    0:00  0.00% darktable/worker res 0
56319   555202  10    0  116M  116M sleep/1   fsleep    0:00  0.00% darktable/worker res 1
56319   520505  10    0  116M  116M sleep/2   fsleep    0:00  0.00% darktable/worker 3
56319   112077  10    0  116M  116M sleep/1   fsleep    0:00  0.00% darktable/worker res 2