btrace(8): add initial support for cli arguments

btrace(8): add initial support for cli arguments

This adds initial support for passing cli args to btrace(8) for use
in bt(5) scripts. Similar to bpftrace, they are referenced via $1,
$2, etc. with $# being the number of command line arguments provided.

Adds an initial regress test and a Makefile change to allow providing
arguments to regress tests in a .args file.

Currently no limit is imposed on the number of arguments, keeping
a similar approach as observed in bpftrace. References to undefined
arguments result in a new "nil" type that contextually acts as a
zero or empty string. More work can be done here to improve bpftrace
compatibility.

ok mpi@, jasper@