Support override of binary name

[dnephin]

This is necessary to support moby/moby#34226. The binary name docker-proxy is currently hardcoded in libnetwork. This is the smallest possible change to allow us to configure the binary name, by setting the following arg on go build of the engine binary:

--ldflags "-X github.com/docker/docker/vendor/github.com/docker/libnetwork/portmapper.userlandProxyCommandName=$PROXY_BINARY_NAME"

[dnephin]

cc @mavenugo @tiborvass

[codecov-io]

Codecov Report

Merging #2007 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2007      +/-   ##
==========================================
+ Coverage   38.01%   38.07%   +0.05%     
==========================================
  Files         137      137              
  Lines       27362    27362              
==========================================
+ Hits        10402    10417      +15     
+ Misses      15688    15674      -14     
+ Partials     1272     1271       -1

Impacted Files	Coverage Δ
portmapper/proxy.go	36.23% <ø> (ø)	⬆️
iptables/iptables.go	54.57% <0%> (+0.85%)	⬆️
networkdb/cluster.go	48.7% <0%> (+2.39%)	⬆️

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[thaJeztah]

LGTM