When running in a non-UTF-8 locale, if an error occurs while importing a
native Python module (say because a dependent share library is missing),
the error message string returned may contain non-ASCII code points
causing a UnicodeDecodeError.

PyUnicode_DecodeFSDefault is used for buffers which may contain
filesystem  paths. For consistency with os.strerror(),
PyUnicode_DecodeLocale is used for buffers which contain system error
messages. While the shortname parameter is always encoded in ASCII
according to PEP 489, it is left decoded using PyUnicode_FromString to
minimize the changes and since it should not affect the decoding (albeit
_potentially_ slower).

In dynload_hpux, since the error buffer contains a message generated
from a static ASCII string and the module filesystem path,
PyUnicode_DecodeFSDefault is used instead of PyUnicode_DecodeLocale as
is used elsewhere.

* bpo-41894: Fix bugs in dynload error msg handling

For both dynload_aix and dynload_hpux, properly handle the possibility
that decoding strings may return NULL and when such an error happens,
properly decrement any previously decoded strings and return early.

In addition, in dynload_aix, ensure that we pass the decoded string
*object* pathname_ob to PyErr_SetImportError instead of the original
pathname buffer.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>