Fix spawning Tor subprocess when datadir contains spaces

[MaximeMRF]

Bugfix, it resolve the issue #351

Motivation & Problem

Spawning the Tor subprocess fails when the data directory path contains spaces:

1. subprocess::Popen was initialized with a concatenated command string, causing internal util::split() to split paths containing spaces into invalid separate arguments.
2. The autogenerated Tor configuration file wrote DataDirectory and ControlPortWriteToFile paths without quotes, causing Tor's config parser to fail on paths with spaces.

Solution

• Pass arguments as std::vector<std::string> to subprocess::Popen so paths with spaces are passed monolithically to the OS process without string splitting.
• Enclose DataDirectory and ControlPortWriteToFile paths in double quotes in generated_config.
• Add subprocess_vector_args_with_spaces unit test in torcontrol_tests.cpp.

[kwsantiago]

The Popen change is the right fix for #351. The other two parts should come out. Details inline.

[kwsantiago]

tACK df4fc30

Verified on Linux with tor 0.4.8.10: datadir containing spaces launches Tor and advertises an onion, -torexecute="env tor" still works so the multi-word case is preserved, plain datadir unaffected, torcontrol_tests green. Generated torrc is back to unquoted values, so the Windows escape-sequence problem is gone.

Nit, not blocking: commit message is the branch name. Something like torcontrol: Pass Tor arguments as a vector to handle paths with spaces would read better in the log.

[chrisguida]

ACK df4fc30

A few nits:

• The PR description is out of date and should be updated to remove the torrc quoting and the subprocess_vector_args_with_spaces test
• The commit message @kwsantiago suggested is better than the current one

Suggestions for follow-up:

• A regression test would actually be easier than @kwsantiago suggested, torcontrol_tests.cpp already forward-declares SplitTorReplyLine/ParseTorReplyMapping to test internals without a header, so hoisting the argv assembly into a small free function would make it directly testable. It would need an #ifdef ENABLE_TOR_SUBPROCESS guard and a bitcoin-build-config.h include in the test, since that's where subprocess.h comes in.
• The tor executable path still can't contain spaces (-torexecute="C:\Program Files\Tor\tor.exe" still breaks, and ENABLE_TOR_SUBPROCESS is on by default on Windows).