unconditionally pass through std/os and deprecate shim/os

[tersec]

Nim 1.2 and newer unconditionally define and document getCurrentProcessId:

proc getCurrentProcessId*(): int {.noWeirdTarget.} =
  ## Return current process ID.
  ##
  ## See also:
  ## * `osproc.processID(p: Process) <osproc.html#processID,Process>`_
  when defined(windows):
    proc GetCurrentProcessId(): DWORD {.stdcall, dynlib: "kernel32",
                                        importc: "GetCurrentProcessId".}
    result = GetCurrentProcessId().int
  else:
    result = getpid()

which adds some comments, uses result = foo, and explicitly checks via .noWeirdTarget. for some cases in which the nim-stew shim won't work either, but is otherwise functionally equivalent.