lib/sh/util.sh: logging: log_level definition: replaced command with message because not just commands can be logged

lib/sh/util.sh: logging: log_level definition: emphasized that the log_level starts with 1, not 0 (0 has a special meaning, which refers to unfiltered messages)

lib/sh/util.sh: logging: log_level definition: documented that for commands that use can_log(), the log_level starts with 1. for unfiltered commands, the log_level is 0 (i.e. still output at verbosity=0).

lib/sh/util.sh: logging: added definition that the log_level is the minimum verbosity needed to display a command

lib/sh/util.sh: set_global_fds(): documented that the logging output for this is visible at verbosity >= 3

lib/sh/util.sh: set_global_fds(): moved stdlog fd setup to separate setup_log_fd() function, which runs at a much higher log_level (5 instead of 3). because the stdlog fd setup statements themselves are logged to fd 2 (because fd 30 is not yet open), you normally don't want to display these, if you are using fd 2 for errors only and fd 30 for logging.

lib/sh/util.sh: command(): $cmd_log_fd: if not set, assume it's a script using util.sh (i.e. with log fd=$log_fd)

lib/sh/util.sh: set_global_fds(): also open stdlog logging stream on fd 30, and use this in the logging functions by setting $log_fd

lib/sh/util.sh: moved streams section before external commands echoing because command() uses set_fds

lib/sh/util.sh: moved echo_eval() to general command echoing section so it can be used by other echoing commands