fix various typos in comment, documentation and log messages.

Add tests for btowc(3)/wctob(3) and enable compilation of the test for
digittoint(3).

The digittoint(3) test is skipped since we don't provide that function yet.

One of the test cases for btowc(3)

Add tests for btowc(3)/wctob(3) and enable compilation of the test for
digittoint(3).

The digittoint(3) test is skipped since we don't provide that function yet.

One of the test cases for btowc(3) is also skipped, since it tests conversion
to Unicode---whereas our wchar_t representation is locale-dependent.

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Add test cases for sprintf/sscanf/strto{d,l} and the is* and isw* ctype functions, for single-byte encodings