lib/sh/util.sh: log_info(): changed symbol to : so that it blends in better with the | lines (looking like a dotted segment of the line), but still stands out (because of the dots)

*{.sh,run}: don't use aliases unless it's actually necessary (e.g. to define a var in the calling function, or to alias-expand the next word)

lib/runscripts/util.run: $top_file: also match dir-level runscripts by removing /run suffix

lib/sh/util.sh: shadow_fd(): take a dest fd instead of a prefix to prepend to the src fd, so that the full name of the dest fd is in the function call. this makes it much clearer what fd will be created from a shadow_fd() call.

lib/sh/util.sh: rel_path(): also remove base_dir when path exactly = to it. this involves adding and removing an extra trailing /.

bugfix: lib/sh/util.sh: rel_path(): ensure non-empty by using . if empty

lib/sh/util.sh: split canon_rel_path() into itself and rel_path() helper for clarity

lib/sh/local.sh: $root_dir: run canon_rel_path on it

lib/sh/util.sh: log_custom(): use keyword-args syntax instead of local vars

lib/sh/util.sh: log_custom(): replace all non-whitespace chars in PS4 with $symbol instead of just the first one, so there aren't mixed $symbols and PS4 chars