add source:line-for-user, returning a 1-indexed line number

add source:line-for-user, returning a 1-indexed line number

* module/system/vm/program.scm (source:line-for-user): New exported
  procedure, returns a 1-indexed line, suitable for presentation to a
  user.
  (write-program): Use source:line-for-user when making fallback names.

* module/system/vm/coverage.scm (coverage-data->lcov):
* module/language/assembly/disassemble.scm (source->string):
* module/system/repl/debug.scm (print-frame): Use source:line-for-user.