CONTRIB: move some dev-specific tools to dev/

The following directories were moved from contrib/ to dev/ to make their
use case a bit clearer. In short, only developers are expected to ever
go there. The makefile was updated to build and clean from these ones.

base64/  flags/  hpack/  plug_qdisc/  poll/  tcploop/  trace/
diff --git a/.gitignore b/.gitignore
index fdbc66a..0e2f78a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,6 +20,7 @@
 !/VERDATE
 !/VERSION
 !/contrib
+!/dev
 !/doc
 !/ebtree
 !/examples
@@ -37,18 +38,18 @@
 *.orig
 *.bak
 # And reject some specific files
-/contrib/base64/base64rev
 /contrib/halog/halog
 /contrib/ip6range/ip6range
 /contrib/iprange/iprange
 /contrib/systemd/haproxy.service
 /contrib/spoa_example/spoa
-contrib/debug/flags
-contrib/poll/poll
+dev/base64/base64rev-gen
+dev/flags/flags
+dev/poll/poll
+dev/tcploop/tcploop
+dev/hpack/decode
+dev/hpack/gen-rht
 contrib/mod_defender/defender
 contrib/modsecurity/modsecurity
-contrib/hpack/decode
-contrib/hpack/gen-rht
-contrib/tcploop/tcploop
 /src/dlmalloc.c
 /tests/test_hashes