Use S?SIZE_MAX, not S?SIZE_T_MAX

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]
Remove now-pointless SIZE_MAX stanza from OpenBSD_malloc_linux