refactor scanf core to use common code path for all string formats

refactor scanf core to use common code path for all string formats

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.