If the nnn in relative(nnn) just means a number of lines, this can be
implemented two ways:

 - if the LHS is a source-line coordinate, (if dwarf_query::spec_type equals
   function_file_and_line) you can add nnn to the line number and perform a 
   normal match.

 - otherwise resolve the LHS to a function or address, map to a line number,
   and then add nnn to that line number and behave as though you were given
   a function_file_and_line query.

As for syntax, .relative(N) may be OK, but so could something shoved into the
basic description string:

  .statement("function@file.c+5")
  # note "+" instead of ":" to indicate relative to first line of function

This whole option probably only makes sense for statement probes that include
function names.

Add .statement("function@file.c+N")
* tapsets.cxx (dwflpp::iterate_over_srcfile_lines): 
Add parameter line_type_relative.  
(enum line_t): New.
(dwarf_query::line_type): New.
(dwarf_query::parse_function_spec): Set line_type.