config: allow changing the default root directory

[theedtron]

This pull request adds a flag called global home root which adds all btcd data into one specified path. I've also added code to check for the flag and reassigns config, datadir, rpc & rpccert file paths.
It fixes issue #2106

Expected outcome
btcd --globalhomeroot=/User/Specified/Path
btcctl --globalhomeroot=/User/Specified/Path
This flag will create a Btcd and Btcctl folder in the user specified path and use the path as the default for all data and configs

[coveralls]

Pull Request Test Coverage Report for Build 8798502118

Details

• 0 of 14 (0.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.03%) to 56.861%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
config.go	0	14	0.0%

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.84%
txscript/taproot.go	2	95.98%
peer/peer.go	6	74.16%

Totals
Change from base Build 8789790194:	-0.03%
Covered Lines:	29451
Relevant Lines:	51795

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💛 - Coveralls