bugfix: lib/sh/util.sh: $top_script: need to realpath this before doing any cd so this points to the right place. set and use $top_script_abs for this purpose.

lib/runscripts/util.run: added setting of $top_file, $top_filename to set_paths()

lib/runscripts/util.run: set $top_file, $top_filename in a function (set_paths__util_run())

lib/sh/util.sh: cd(): re-run set_paths() to update $top_script, $top_dir for the new current dir

lib/sh/util.sh: added set_paths(), which sets $top_script, $top_dir

lib/sh/util.sh: moved $top_script, $top_dir before internal commands echoing so they can be reset in cd()

lib/sh/util.sh: moved vars echoing before internal commands echoing so that echo_vars can be used in it

lib/sh/util.sh: use set_fds() instead of `echo_eval exec` for clarity

lib/sh/util.sh: set_fd(): allow setting multiple fds at once, and changed the function name accordingly

lib/sh/util.sh: set_fd(): moved noclobber functionality to fd_set_default() since that is only used by fd_set_default() (whose purpose has become just to run set_fd without clobbering an existing dest fd)