bugfix: lib/sh/util.sh: fwd(): need to use $top_dir (the runscript's file) instead of "${BASH_SOURCE¹}" (the caller's file)

lib/sh/util.sh: command__set_fds(): cmd_log_fd = log_fd and can_log: don't call set_fds because no redirection is needed. this also avoids the need to increase the log_level of the set_fds call that used to set cmd_log_fd to itself.

bugfix: lib/sh/util.sh: command__set_fds(): cmd_log_fd = log_fd: need to increase the log_level after can_log is checked, to avoid limiting the command output itself

bugfix: lib/sh/util.sh: command__set_fds(): log the command output at the same log_level as the command name (i.e. leave the log_level as-is), to allow nesting verbose commands within one another without the inner commands always getting suppressed

lib/sh/util.sh: command__set_fds(): set the src fd outside of the set_fds call, for clarity

lib/sh/util.sh: command(): if cmd_log_fd = log_fd, log the filtering of cmd_log_fd at 2 log_levels higher (verbosity >= 3)

lib/sh/util.sh: command(): documented that $cmd_log_fd's default (no cmd_log_fd) limits log_fd in case command uses util.sh

bugfix: lib/sh/util.sh: command(): need to make $cmd_log_fd a local var when setting its default value

lib/sh/util.sh: logging: moved indent for call tree to its own section, so it isn't confused with the log_level

lib/sh/util.sh: logging: log_level definition: added instructions to view a message's log_level (count the # of + signs before it in the output)