lib/sh/util.sh: redir(): override save_e and add `unset redirs` so error handlers are not redirected

lib/sh/util.sh: added alias_append(), similar to func_override() for aliases

lib/sh/util.sh: redir(): don't do redir actions if redir will be run later (i.e. if command to run starts with `redir` or `command`)

lib/sh/util.sh: redir(), command(): log their function calls at the usual log_level (2) instead of one higher, so that they are printed in the call tree to help debugging

lib/sh/util.sh: added redir() and use it in command() to perform and echo the redirections

lib/sh/util.sh: command(): removed comment that "the following redirections must happen in exactly this order", because there is now only one redirection

lib/sh/util.sh: command(): removed 2>&$err_fd, which is no longer needed because $err_fd is now 2 (in general, there is probably not a need for a special $err_fd var, because 2 is already stderr)

lib/sh/util.sh: command(): don't set err_fd to global stderr, because this prevents errors from being captured and parsed by callers. it is also not necessary to use a separate port than stderr, because stderr already contains only errors (since logging messages go to their own port). global stderr would still be useful e.g. for displaying input prompts the user when reading from global stdin.

bugfix: *run: overriding targets: use new self_make to properly progagate the $remake flag to the overridden target, so that the target itself is not skipped

lib/sh/util.sh: to_file(): added del= flag to prevent the file from being auto-removed on error (e.g. to preserve a partial result, which normally would be removed)