lib/sh/util.sh: streams: moved setting of global stdin/stdout/stderr into set_global_fds () function

lib/sh/util.sh: streams: allow commands to access global stdin/stdout/stderr using fd 20/21/22. this works even when /dev/tty isn't available.

lib/sh/util.sh: streams: added convention that fd 10/11/12 should be used for command-specific alternate stdin/stdout/stderr

lib/sh/util.sh: added streams functions fd_exists, set_fd, shadow_fd, and helpers

lib/sh/util.sh: added echo_minor_func alias

lib/sh/util.sh: added echo_eval()

lib/sh/util.sh: renamed require_exists() to require_not_exists() because the command actually checks if the file doesn't exist

lib/sh/util.sh: echo_vars(): use log() instead of manually echoing the log indent, so that the log indent string ($log_indent$PS4) doesn't have to be maintained in several different places

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.