Handle and use Linux capabilities, logging improvements, cleanup, documentation

This MR is somewhat a successor to !1812 (closed) in that it can allow the softmodem(s) (tested: 5G) to run without root privileges. However, no explicit option is necessary to do this. More generally, in this MR we check for some capabilities (notably, SYS_NICE) and handle some errors (for NET_ADMIN) to allow this. However, the scope of this MR is wider, in that it attempts to make use and handle Linux capabilities (see capabilities(7)), in particular for containerized workloads. More concretely:

• Remove many attempts to change or workarounds around host system configuration:
  • Remove set_latency_target(), which sets DMA latency target and modifies CPU scaling, and page locking: this might not be allowed or possible inside a container
  • Remove attempt to change kernel socket buffer sizes in RFsim, and put it into documentation
  • don't check for fedora (completely unclear why)
• Handle capabilities, and harmonize code:
  • check for SYS_NICE (either via libcap, or a workaround if not present) and only change thread affinity/priority if present -> allows gNB to run without privileges, UE partially
  • handle missing IPC_LOCK gracefully (but print a warning)
  • handle problems with NET_ADMIN (interface creation, configuration) gracefully instead of stopping softmodem -> allows UE and gNB to run without privileges
  • use threadCreate() with OAI_PRIORITY_RT consistently in (n)FAPI code
• Correctly set capabilities in all docker-compose files
• Add documentation on:
  • performance tuning and capabilities
  • physical simulators (not directly linked to this MR, but still)
• Cleanup and fixes:
  • Reduce amount of useless logging from ITTI, config, utils for threading, RFsim, L1
  • fix a bug in CI where the wrong image might be used for unit tests

Labels 5G-NR and 4G-LTE because while the focus is on 5G, there is common code with 4G as well

Follow-up work will go into better define, configure, and document threads (notably ITTI, RLC, PDCP).