correctly handle write errors encountered by printf-family functions

previously, write errors neither stopped further output attempts nor
caused the function to return an error to the caller. this could
result in silent loss of output, possibly in the middle of output in
the event of a non-permanent error.

the simplest solution is temporarily clearing the error flag for the
target stream, then suppressing further output when the error flag is
set and checking/restoring it at the end of the operation to determine
the correct return value.

since the wide version of the code internally calls the narrow fprintf
to perform some of its underlying operations, initial clearing of the
error flag is suppressed when performing a narrow vfprintf on a
wide-oriented stream. this is not a problem since the behavior of
narrow operations on wide-oriented streams is undefined.
add basic dns record parsing functions

based on patch by Timo Teräs, with some corrections to bounds checking
code and other minor changes.

while they are borderline scope creep, the functions added are fairly
small and are roughly the minimum code needed to use the results of
the res_query API without re-implementing error-prone DNS packet
parsing, and they are used in practice by some kerberos related
software and possibly other things. at this time there is no intent to
implement further nameser.h API functions.
implement FNM_CASEFOLD extension to fnmatch function
provide CMPLX macros in implementation-internal libm.h

this avoids assuming the presence of C11 macro definitions in the
public complex.h, which need changes potentially incompatible with the
way these macros are being used internally.
fix definition of CMPLX macros in complex.h to work in constant expressions

based on patches by Jens Gustedt. these macros need to be usable in
static initializers, and the old definitions were not.

there is no portable way to provide correct definitions for these
macros unless the compiler supports pure imaginary types. a portable
definition is provided for this case even though there are presently
no compilers that can use it. gcc and compatible compilers provide a
builtin function that can be used, but clang fails to support this and
instead requires a construct which is a constraint violation and which
is only a constant expression as a clang-specific extension.

since these macros are a namespace violation in pre-C11 profiles, and
since no known pre-C11 compilers provide any way to define them
correctly anyway, the definitions have been made conditional on C11.
make the result of the cimag macro a non-lvalue

this change is not necessary but helps diagnose invalid code. based on
patch by Jens Gustedt.
make the definition of _Complex_I explicitly complex

it's unclear whether compilers which provide pure imaginary types
might produce a pure imaginary expression for 1.0fi. using 0.0f+1.0fi
ensures that the result is explicitly complex and makes this obvious
to human readers too.
fix signedness of WINT_MIN expression

since wint_t is unsigned, WINT_MIN needs to expand to an unsigned zero.