fix: unreachable supernode ip in p2p-network

[lowzj]

Signed-off-by: lowzj [email protected]

Ⅰ. Describe what this PR did

Make it possible for users to set the supernode's ip that we advertiese to other peers in the p2p-network, the property is: advertieseIp

java -Dsupernode.advertieseIp=10.1.1.1 -jar supernode.jar

By default, the first non-loop address of the host is advertied.

Ⅱ. Does this pull request fix one issue?

fixes: #252

[codecov-io]

Codecov Report

Merging #277 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
- Coverage   58.89%   58.73%   -0.16%     
==========================================
  Files          56       36      -20     
  Lines        2788     1854     -934     
==========================================
- Hits         1642     1089     -553     
+ Misses       1068      723     -345     
+ Partials       78       42      -36

Impacted Files	Coverage Δ
dfget/core/uploader/uploader.go	25% <0%> (-69.18%)	⬇️
...core/downloader/back_downloader/back_downloader.go	0% <0%> (-59.19%)	⬇️
dfget/core/uploader/uploader_util.go	67.03% <0%> (-30.03%)	⬇️
dfget/core/downloader/downloader.go	72.09% <0%> (-27.91%)	⬇️
dfget/errors/errors.go	37.03% <0%> (-6.45%)	⬇️
cmd/dfget/app/root.go	61.87% <0%> (-5.51%)	⬇️
cmd/dfget/app/server.go	55.55% <0%> (-4.45%)	⬇️
dfget/core/api/uploader_api.go	0% <0%> (ø)	⬆️
dfdaemon/util/urlutil.go	100% <0%> (ø)	⬆️
dfget/core/helper/test_helper.go	0% <0%> (ø)	⬆️
... and 32 more

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[starnop]

LGTM.