bugfix: lib/runscripts/import.run: all(): also need to propagate $rm to import()

bugfix: lib/runscripts/table.run: table_make_install(): need to ignore skip_table() errexit

added lib/runscripts/extract_header.run

fix: lib/runscripts/util.run: to_top_file(): handle $_remake properly, without requiring deferred_check_target_exists to set to_file()'s flags

*{.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.)

lib/sh/make.sh: self_make(): renamed to with_rm() for clarity, since this is used only to progagate $rm, and does not also invoke a command with the same name as the current function, as the name might suggest

lib/runscripts/util.run: support conventional main() method as well as `all` target

bugfix: lib/runscripts/table.run: schema/make calls: need to use `make schema` instead because old-style datasources don't have a top-level runscript (the absence of this identifies them as old-style so inputs/input.Makefile works correctly)

lib/runscripts/util.run: on_exit(): documented that users can also override gateway()/fallback() to perform other commands (or no commands) after the script is read

fix: lib/runscripts/table.run: table_make_install(): need to inform the user when it skips installing a table, because this is often unexpected