Restrict rewrite of unary operators to dereference operator

Restrict rewrite of unary operators to dereference operator

Since the whole expression, unary operator included, is replaced by a
call to bpf_probe_read, the dereference operator is currently the
only unary operator properly rewritten. When rewriting an increment
expression (++val) for instance, the increment operator is lost in
translation.

Trying to rewrite all unary operators sometimes confuses bcc and
results in improper code, for instance when trying to rewrite a
logical negation.