Argument Clinic: move converters defined in several files to libclinic

Several custom converters are defined in more than one C file:

• pid_t — in posixmodule.c, _posixsubprocess.c and resource.c.
• Py_off_t — in posixmodule.c and _ssl.c; the Py_off_t type and the converter function are duplicated as well, and _io has yet another copy of the type.
• HANDLE — in _multiprocessing/multiprocessing.c, overlapped.c, msvcrtmodule.c, _testconsole.c and _winapi.c.
• DWORD — in overlapped.c, winreg.c and _winapi.c.
• BOOL — in overlapped.c and _winapi.c.

The copies are not always equivalent: two of the pid_t converters compare the result with -1 instead of (pid_t)-1, and DWORD in _winapi.c uses the format unit "k", which accepts negative values, while in other modules it rejects them.

The registry of converters is global, so the definition in the file which is processed first can be used in other files. The generated code therefore depends on the order of processing — removing the definition of HANDLE from one module silently changes the code generated for others.

Other converters are defined only in posixmodule.c, although the corresponding C functions are already shared, so other modules convert the argument in the "impl" function instead of declaring its type:

• uid_t and gid_t — _Py_Uid_Converter() and _Py_Gid_Converter() are declared in posixmodule.h and are called by hand in pwd.getpwuid(), grp.getgrgid() and _posixsubprocess.fork_exec().

There is no converter for time_t at all: time.gmtime(), time.localtime() and time.ctime() parse their argument with a hand-written helper, and _datetime calls _PyTime_ObjectToTime_t() directly.

I am going to move these converters to Tools/clinic/libclinic/converters.py.

Linked PRs

• gh-156262
• gh-156294

Start in Tools/clinic/libclinic/converters.py, then inspect the converter definitions and shared declarations in posixmodule.c, _posixsubprocess.c, resource.c, _ssl.c, _io, and the Windows modules listed in the issue. Check how Argument Clinic processes modules and compare generated code; done means the converters are centralized without changing their accepted values or generated behavior.