Don't use global (external) symbols for symbols that are used in only one translation unit.

Before:

```
$ for SYMBOL in $(nm src/bitcoind | grep -E ' [BD] ' | c++filt | cut -f3- -d' ' | grep -v @ | grep -v : | sort | grep '[a-z]' | sort -u | grep -vE '(^_|typeinfo|vtable)'); do
      REFERENCES=$(git grep -lE "([^a-zA-Z]|^)${SYMBOL}([^a-zA-Z]|\$)" -- "*.cpp" "*.h")
      N_REFERENCES=$(wc -l <<< "${REFERENCES}")
      if [[ ${N_REFERENCES} > 1 ]]; then
          continue
      fi
      echo "Global symbol ${SYMBOL} is used in only one translation unit: ${REFERENCES}"
  done
Global symbol g_chainstate is used in only one translation unit: src/validation.cpp
Global symbol g_ui_signals is used in only one translation unit: src/ui_interface.cpp
Global symbol instance_of_cmaincleanup is used in only one translation unit: src/validation.cpp
Global symbol instance_of_cnetcleanup is used in only one translation unit: src/net.cpp
Global symbol instance_of_cnetprocessingcleanup is used in only one translation unit: src/net_processing.cpp
Global symbol pindexBestForkBase is used in only one translation unit: src/validation.cpp
Global symbol pindexBestForkTip is used in only one translation unit: src/validation.cpp
$
```

After:

```
$ for SYMBOL in $(nm src/bitcoind | grep -E ' [BD] ' | c++filt | cut -f3- -d' ' | grep -v @ | grep -v : | sort | grep '[a-z]' | sort -u | grep -vE '(^_|typeinfo|vtable)'); do
      REFERENCES=$(git grep -lE "([^a-zA-Z]|^)${SYMBOL}([^a-zA-Z]|\$)" -- "*.cpp" "*.h")
      N_REFERENCES=$(wc -l <<< "${REFERENCES}")
      if [[ ${N_REFERENCES} > 1 ]]; then
          continue
      fi
      echo "Global symbol ${SYMBOL} is used in only one translation unit: ${REFERENCES}"
  done
$
```

♻️ Think about future generations: save the global namespace from unnecessary pollution!  ♻️

Empact:
  ACK https://github.com/bitcoin/bitcoin/pull/16092/commits/0959d37e3e0f80010a78d175e3846dabf5d35919
MarcoFalke:
  ACK 0959d37e3e0f80010a78d175e3846dabf5d35919
hebasto:
  ACK 0959d37e3e0f80010a78d175e3846dabf5d35919
promag:
  ACK 0959d37.