This change restores the .symtab symbol table files in our flagship
programs (e.g. redbean.com, python.com) needed to show backtraces. This
also rolls back earlier changes to zip.com w.r.t. temp directories since
the right way to do it turned out to be the -b DIR flag.
This change also improves the performance of zip.com. It turned out
mmap() wasn't being used, because zip.com was assuming a 4096-byte
granularity, but cosmo requires 65536. There was also a chance to speed
up stdio scanning using the unlocked functions.
