This fixes bug 13102 (not on any released Tor) where using the
standard SSIZE_MAX name broke mingw64, and we didn't realize.
I did this with
perl -i -pe 's/SIZE_T_MAX/SIZE_MAX/' src/*/*.[ch] src/*/*/*.[ch]
The rationale for treating these files differently is that we should
be checking upstream for changes as applicable, and merging changes
upstream as warranted.