Add option to limit the maximum number of AST nodes
Add option to limit the maximum number of AST nodes

When doing fuzzing, it is important to limit the number of AST nodes
because otherwise a fuzzer might keep to generate a very long program
that cause a stack overflow.

This introduces `BPFTRACE_NODE_MAX` environment variable, and it
controls the maximum number of AST nodes. This feature is intended for
developers, and its usage is not included in the reference guide.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace