Trying to reason about error propagation and resource leakage within
the form submission code was impossible because of the
form_successful_controls_dom function.
This function was over six hundred lines long, had twenty six top
level local variables and six levels of indent in places.
This commit splits it out into thirteen shorter and more obvious
functions. The resulting operation is identical except errors are
properly propagated (all failures were reported as out of memory)
and resource management can be reasoned about.
The compiler appears to inline the entirety of the code from
form_submit() down excepting a handful of leaf functions. This
results in similar code output size as previous implementation.
The new implementation has a greater number of variables passed to sub
functions than desirable because multiple character sets are required
to encode names and values in the multipart data list. However as
noted the compiler effectively inlines all these functions so this
does not actually become a major problem.
