memory, out of disk space, or test big directories.

Support -q option to run only the quick tests, which saves about
7mins.

Clean up driver by removing duplicated code.

can run.

Must assumed_free in badwrite() be adjusted?

Use the flags in elf header to set vm permissions
Modify pgbug() so that usertests text segment is without W
Add test to check app cannot write text segment

Growing adds R|W pages (without X).  Exec() marks the stack only R|W.

Probably could setup permissions for text and data better if we call
ld with --no-omagic instead of -N.