Reduce compiler warnings

[ofalk]

As discussed in #63 - there are additional warnings that should be addressed:

• -Wstringop-overflow=
• -Wunused-variable
• -Wunused-function

As of 2023-11-06:

# rpmbuild -ba --with check libdnet.spec --quiet 2>&1 | grep -- -W | sed 's/.*\[//; s/\]//' | sort | uniq -c | sort -nr
     34 -Wpointer-sign
      8 -Wincompatible-pointer-types
      2 -Wsizeof-pointer-memaccess
      1 -Wunused-variable
      1 -Wunused-result
      1 -Wunused-function
      1 -Wstringop-overflow=
      1 -Wmaybe-uninitialized
      1 -Wimplicit-function-declaration
      1 -Warray-bounds