Windows: CreateProcessA() doesn't use lpCurrentDirectory to understand lpCommandLine

[lucasart]

Possible fixes:

• reconstruct a path relative to the parent's cwd: https://github.com/nkg114mc/c-gomoku-cli/pull/4/commits/13244964a2569e35872b61cc42e7d34e45e25cb3.
• chdir(cwd) before CreateProcessA(). This replicates the POSIX code logic.

[lucasart]

Actually, the chdir() solution is best, as it also avoids this problem: https://github.com/nkg114mc/c-gomoku-cli/commit/f14def755f997911b0f7325da3d9a2c7a775bf60.

[lucasart]

chdir() will be done in the context of the parent unlike on POSIX. So this must be locked as in #52.