Rename `usemem.c` to `oom.c`.
oom.c: Add usage message and `-h`/`--help` option.
oom.c: Fix segmentation fault error.
oom.c: Use different messages for the two out-of-memory conditions that can occur.
oom.c: Rename option `--brief` to `--quiet`.
oom.c: Add option `-p`/`--processes` to fork a specified number of processes.

This lets one use an arbitrarily large quantity of memory, regardless of
VM overcommit settings.