Fix with-bpf-session crash when used from CL-USER

Two bugs fixed:

1. session.lisp: bpf:prog body symbols from CL-USER (e.g. CL:INCF,
   CL-USER::GETMAP) were not re-interned into the WHISTLER package,
   so CL's INCF macroexpander fired instead of Whistler's, producing
   forms the lowerer could not handle. Added whistler-intern-form to
   re-intern body symbols before compilation.

2. lower.lisp: the unbound-variable error handler called (search "%%"
   sym) on a symbol instead of a string, triggering a TYPE-ERROR that
   masked the real diagnostic. Fixed to use symbol-name.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>