osproc: fix double close on POSIX (#5724)

Calling close() in some cases issued two close() syscalls to one FD, which is incorrect in multithreaded programs.

tests/osproc/tclose.nim

@@ -0,0 +1,24 @@
+discard """
+  exitcode: 0
+"""
+
+when defined(linux):
+  import osproc, os
+
+  proc countFds(): int =
+    result = 0
+    for i in walkDir("/proc/self/fd"):
+      result += 1
+
+  let initCount = countFds()
+
+  let p = osproc.startProcess("echo", options={poUsePath})
+  assert countFds() == initCount + 3
+  p.close
+  assert countFds() == initCount
+
+  let p1 = osproc.startProcess("echo", options={poUsePath})
+  discard p1.inputStream
+  assert countFds() == initCount + 3
+  p.close
+  assert countFds() == initCount