lib/sh/util.sh: rethrow(): also work in situations where $e is not set

lib/sh/util.sh: rethrow: made it a function since there is now no need for it to be an alias

lib/sh/util.sh: rethrow: removed `test "$e" != 0` since errexit only does anything if $e != 0

lib/sh/util.sh: removed separate rethrow_exit*, rethrow_subshell*, since they now do the same thing as rethrow*

lib/sh/util.sh: rethrow*!: use new errexit, which works in functions and subshells

lib/sh/util.sh: added errexit(), used in place of (exit "$1") because a bug in bash prevents subshells from triggering errexit

lib/sh/util.sh: added bool!()

fix: lib/sh/util.sh: redir(): need to indent before invoking an external command (not just in command__exec(), but for all redir() calls)

lib/sh/make.sh: with_rm(): documented that it only works inside a runscript target that starts w/ begin_target

*{.sh,run}: runscript targets: use begin_target instead of echo_func so the target name is properly echoed. note that this requires using with_rm so that $rm is properly progagated to applicable invoked targets. (previously, $rm was progagated to all invoked targets. note that with_rm only works inside a runscript target that starts with begin_target.)