policy is that all public functions which have a public declaration
should be defined in a context where that public declaration is
visible, to avoid preventable type mismatches.

an audit performed using GCC's -Wmissing-declarations turned up the
violations corrected here. in some cases the public header had not
been included; in others, a feature test macro needed to make the
declaration visible had been omitted.

in the case of gethostent and getnetent, the omission seems to have
been intentional, as a hack to admit a single stub definition for both
functions. this kind of hack is no longer acceptable; it's UB and
would not fly with LTO or advanced toolchains. the hack is undone to
make exposure of the declarations possible.

this patch adjusts libc components which use the multibyte functions
internally, and which depend on them operating in a particular
encoding, to make the appropriate locale changes before calling them
and restore the calling thread's locale afterwards. activating the
byte-based C locale without these changes would cause regressions in
stdio and iconv.

in the case of iconv, the current implementation was simply using the
multibyte functions as UTF-8 conversions. setting a multibyte UTF-8
locale for the duration of the iconv operation allows the code to
continue working.

in the case of stdio, POSIX requires that FILE streams have an
encoding rule bound at the time of setting wide orientation. as long
as all locales, including the C locale, used the same encoding,
treating high bytes as UTF-8, there was no need to store an encoding
rule as part of the stream's state.

a new locale field in the FILE structure points to the locale that
should be made active during fgetwc/fputwc/ungetwc on the stream. it
cannot point to the locale active at the time the stream becomes
oriented, because this locale could be mutable (the global locale) or
could be destroyed (locale_t objects produced by newlocale) before the
stream is closed. instead, a pointer to the static C or C.UTF-8 locale
object added in commit commit aeeac9ca
is used. this is valid since categories other than LC_CTYPE will not
affect these functions.

when the orientation of the stream was already set, fwide was
incorrectly returning its argument (the requested orientation) rather
than the actual orientation of the stream.

these functions must behave as if they obtain the lock via flockfile
to satisfy POSIX requirements. since another thread can provably hold
the lock when they are called, they must wait to obtain the lock
before they can return, even if the correct return value could be
obtained without locking. in the case of fclose and freopen, failure
to do so could cause correct (albeit obscure) programs to crash or
otherwise misbehave; in the case of feof, ferror, and fwide, failure
to obtain the lock could sometimes return incorrect results. in any
case, having these functions proceed and return while another thread
held the lock was wrong.