This macro block encapsulates the common pattern of switching to the prior (formerly called old) mem context to return results from a function.
Also rename MEM_CONTEXT_OLD() to memContextPrior(). This violates our convention of macros being in all caps but memContextPrior() will become a function very soon so this will reduce churn.
A recopy would occur if the size or checksum was invalid but on error the backup would terminate.
Instead, recopy the resumed file on any error. If the error is systemic (e.g. network failure) then it should show up again during the recopy.
Adding a dummy column which is always set by the P() macro allows a single macro to be used for parameters or no parameters without violating C's prohibition on the {} initializer.
-Wmissing-field-initializers remains disabled because it still gives wildly different results between versions of gcc.
The local process is now entirely migrated to C. Since all major I/O operations are performed in the local process, the vast majority of I/O is now performed in C.
Contributed by David Steele, Cynthia Shang.
