pthread_mutex_t is a CRITICAL_SECTION (not a pointer) on Win32

Partially revert Deuce's commit b4bc9c97

This explains the EAccessViolation (stack corruption during sbbs startup) that sysops have reported when using the latest nightly builds.
14 jobs for master in 9 minutes and 33 seconds (queued for 17 seconds)
Status Name Job ID Coverage
  Build
passed jsdoor-freebsd #31300
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passed jsdoor-linux #31301
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passed jsdoor-windows #31303
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passed sbbs-freebsd #31295
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passed sbbs-linux #31290
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passed sbbs-windows #31293
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passed sexpots-freebsd #31302
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passed sexpots-linux #31291
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passed sexpots-windows #31294
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passed syncdraw-freebsd #31298
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passed syncdraw-linux #31299
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passed syncterm-freebsd #31296
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passed syncterm-linux #31292
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passed syncterm-windows #31297
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