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

lib/sh/util.sh: log++/-- (and helpers PS4++/--): added functions for these so they can also be used in other contexts that don't support aliases. note that direct callers of the functions would still need to localize the PS4/$verbosity values using log_local.

lib/sh/util.sh: run_args_cmd(): moved test of $? to lib/runscripts/util.run on_exit() since it needs to be performed for all run_args_cmd() functions, not just the default implementation. (this test ensures that no commands are executed if the EXIT trap was encountered due to a parsing/loading error).

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