lib/sh/util.sh: logging: visually separate the log_level-based indenting from the external command indenting by using the standard `set -x` prefix (1st char of PS4) for log_level indenting and $log_level_indent (i.e. whitespace) for external command indenting

lib/sh/util.sh: logging: separate the log_level-based indenting from the external command indenting so that the log_level-based indenting can use a different prefix. this involves propagating a new var, $log_indent, to invoked commands instead of $PS4, and always prepending $log_indent to $PS4 when printing log messages.

lib/sh/util.sh: propagate $log_level_indent to invoked commands

lib/sh/util.sh: renamed $log_indent to $log_level_indent to indicate that this is the string added at each level, rather than the entire indent

bugfix: lib/sh/util.sh: limit_stderr(): switched back to running inc_log_level inside limit_stderr(), because it should only apply during the function for use by can_log. documented why inc_log_level needs to be inside limit_stderr().

bugfix: lib/sh/util.sh: limit_stderr(): stdout2stderr mode: only limit stdout, since stderr may contain error messages (which should always be displayed)

bugfix: lib/sh/util.sh: limit_stderr_cmd(): need to load new aliases before it so limit_stderr is expanded

bugfix: lib/sh/util.sh: limit_stderr alias: need to inc_log_level before rather than after limit_stderr(), so that can_log uses the right verbosity (previously, inc_log_level was part of limit_stderr() itself, but was moved to the alias because its effects were needed outside the function)

lib/sh/util.sh: stdout2stderr(): fixed doc comment after space-before-() search-and-replace

lib/sh/util.sh: usage(): use `return` instead of `(exit ...)`