Import build config and DHCP relay code from bpf-examples

The dhcp-relay utility was initially implemented as part of the
bpf-examples repository, but really belongs here. So import it along with
the build environment from bpf-examples.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

.gitignore

@@ -1,52 +1,5 @@
-# Prerequisites
-*.d
-
-# Object files
+config.mk
+.ccls-cache
+compile_commands.json
+*.ll
 *.o
-*.ko
-*.obj
-*.elf
-
-# Linker output
-*.ilk
-*.map
-*.exp
-
-# Precompiled Headers
-*.gch
-*.pch
-
-# Libraries
-*.lib
-*.a
-*.la
-*.lo
-
-# Shared objects (inc. Windows DLLs)
-*.dll
-*.so
-*.so.*
-*.dylib
-
-# Executables
-*.exe
-*.out
-*.app
-*.i*86
-*.x86_64
-*.hex
-
-# Debug files
-*.dSYM/
-*.su
-*.idb
-*.pdb
-
-# Kernel Module Compile Results
-*.mod*
-*.cmd
-.tmp_versions/
-modules.order
-Module.symvers
-Mkfile.old
-dkms.conf