check in configure to be polite (failing early if we're going to fail)
and in vfprintf.c since that is the point at which a mismatching type
would be extremely dangerous.

for conversion specifiers, alloc is always set when the specifier is
parsed. however, if scanf stops due to mismatching literal text,
either an uninitialized (if no conversions have been performed yet) or
stale (from the previous conversion) of the flag will be used,
possibly causing an invalid pointer to be passed to free when the
function returns.

this seems to have been a regression from the refactoring which added
the 'm' modifier.

this commit only covers the byte-based scanf-family functions. the
wide functions still lack support for the 'm' modifier.

this brings the wide version of the code into alignment with the
byte-based version, in preparation for adding support for the m
(malloc) modifier.

the concept here is that %s and %c are essentially special-cases of
%[, with some minimal additional special-casing.

aside from simplifying the code and reducing the number of complex
code-paths that would need changing to make optimizations later, the
main purpose of this change is to simplify addition of the 'm'
modifier which causes scanf to allocate storage for the string being
read.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.