makesyscalls: rip out arbitrary command execution

[kevans]

Summary

This was previously needed only for CloudABI, which used it to generate
its capenabled from syscalls.master. CloudABI was removed in
cf0ee8738e31, so we don't need to support this anymore. Others looking
to do similar things should come up with a more integrated technique,
such as a .conf flag or pattern/glob support.

MFC after: never

[imp]

Kill it with fire

[brooks]

The CloudABI case could have easily been support by D31349 or an extension on top of it to specify always-applied flags.