lib/sh/util.sh: expand symlinks in $PWD: moved it before canon_rel_path(), which will require it

lib/sh/util.sh: expand symlinks in $PWD using `cd -P .` so it matches the output of realpath

lib/sh/util.sh: cd(): documented what -P option does and why it's used

lib/sh/util.sh: cd(): always run cd with -P (expand symlinks) so that $PWD will contain a canonical path which matches the output of `readlink -f`. however, don't display the added -P in the logging output because it distracts from the directory being changed to.

lib/sh/util.sh: logging: added log+/-, which take a variable log_level step, and use them in log++/--

lib/sh/util.sh: repeat(): for simplicity and speed, just append to a local string var (and echo the result at the end) instead of using printf

bugfix: lib/sh/util.sh: repeat(): need to use %s instead of %q (escaped string) in printf

lib/sh/util.sh: logging: log++/--: comment about the different uses can be performed with one function: clarified that without a cmd, "$@" expands to nothing and assignments are applied to the caller

lib/sh/util.sh: log++/--: support running a command with the given log++/-- setting instead of applying it in the calling context. note that with no args, "$@" will expand to nothing and the var assignments will be applied in the calling context instead of to an executed command. this requires using $(()) syntax instead of let! to set the verbosity.

lib/sh/util.sh: logging: inlined PS4 functions into log++/-- because that's the only place they're used and it greatly simplifies the code