GUI/Intro: pass -blocksdir to getPathToCheck if present

[diegoviola]

Fixes: #946

[DrahtBot]

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Concept ACK	pablomartin4btc

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[pablomartin4btc]

Concept ACK

The actual problem in #946 is the intro dialog's prune suggestion (only shown the first time bitcoin-qt runs) — FreespaceChecker checks datadir's free space but the prune trigger uses the full blockchain + chainstate threshold (~880 GB), incorrectly suggesting prune when -blocksdir is set since only ~14 GB of chainstate goes to datadir. Skipping the dialog avoids the wrong suggestion.

[hebasto]

I'm not sure this is the correct approach.

For example, it breaks the -choosedatadir option, which is now ignored when combined with -blocksdir:

$ ./build_dev_mode/bin/bitcoin-qt -choosedatadir -blocksdir=/tmp/test_blocksdir
# no intro dialog is shown

[diegoviola]

I'm not sure this is the correct approach.

For example, it breaks the -choosedatadir option, which is now ignored when combined with -blocksdir:

$ ./build_dev_mode/bin/bitcoin-qt -choosedatadir -blocksdir=/tmp/test_blocksdir
# no intro dialog is shown

Can you please try this new approach? The dialog is now being shown for me and prune is disabled when running ./bin/bitcoin-qt -choosedatadir -blocksdir=/path/to/bitcoin-blocks/ which I think is the correct behavior.