lib/sh/util.sh: moved canon_rel_path () into separate paths section

bugfix: : use new func_override for runscript inheritance instead of invoking the overridden function as a (command-line) target of the parent runscript. this ensures that the overridden function is run in the *same process as the calling function, so that $top_dir keeps the same value and runscript-relative paths continue to work.

lib/sh/util.sh: added func_override (), for use in runscript inheritance

lib/sh/util.sh: copy_func (): check that $from exists and $to does not exist (i.e. don't clobber existing functions). you can always first `unset -f` the function to get around the no-clobber restriction.

lib/sh/util.sh: exceptions: added die ()

lib/sh/util.sh: log_e (): restructured as an error handler to put after save_e rather than as a wrapper around the entire command. this allows it to be used with any kind of expression (such as boolean expressions with !), not just single commands.

lib/sh/util.sh: log_e (): don't export $e to the calling context, since the caller can just use ` || { save_e; ...; }` if they need $e

lib/sh/util.sh: calls to log_e: don't rely on log_e setting $e

lib/sh/util.sh: added save_e, which now sets $e just locally

lib/sh/util.sh: save_e: renamed to export_e because $e overwrites any previous value in the calling context