posix: Disable debug output of pthreads

`DEBUG_ENABLED` should be enabled during debugging, and disabled
afterwards.
Fix test_net_if
Merge pull request #848 from Kijewski/that-annoying-debug-flag-in-pthread-that-effing-needs-to-go-away-already

posix: Disable debug output of pthreads
Merge pull request #853 from authmillenon/fix-test-net-if

tests: Fix test_net_if
shell: remove clutter from shell's Makefile

Remove clutter that was put into centralized Makefile.includes.
Merge pull request #857 from Kijewski/shell-remove-clutter

shell: remove clutter from shell's Makefile