lib/sh/util.sh: added unexport() (note that `declare +x` won't work because it defines the var if it isn't set)

lib/sh/util.sh: moved vars section to beginning so other sections can use it (it does not contain logging statements, so it doesn't need to be after verbose output)

bugfix: lib/sh/util.sh: command(): limit cmd_log_fd directly instead of limiting log_fd (usually fd 2) and then redirecting cmd_log_fd to it. this ensures that stderr is not limited unless it's actually used as the cmd_log_fd, so that a non-logging stderr will still reach the user properly regardless of whether err_fd is set to something other than 2. (previously, without an err_fd at 22, closing fd 2 at the beginning of the set of redirections would leave no fd pointing to a place to echo errors to.)

lib/sh/util.sh: disable_logging(): use echo_eval instead of plain eval so the disabling of logging is nevertheless itself logged

lib/sh/util.sh: command(): made usage message an end-of-line comment

lib/sh/util.sh: inlined limit_log_fd into command() since it's only used once

lib/sh/util.sh: log_err(), log_info(): use bolder symbols (*, #) to make the symbol stand out visually from the line of | above and below it

lib/sh/util.sh: $verbosity: lowered the default from 3 to 2 because level 3 output (vars, etc.) is too verbose for the default output and is only needed for debugging

lib/sh/util.sh: $log_level_indent: added space after | to visually separate the nesting levels and make the output look like an expandable tree (similar to a filesystem browser tree)

lib/sh/util.sh: log_custom(): replace every non-whitespace character of $log_indent, and the first character of PS4, with the symbol, to make the symbol stand out on the line instead of the |||... before it