units: simplify rescue.service and emergency.service (#5623)

The emergency.service and rescue.service units have become rather
convoluted. We spawn multiple shells and the help text spans multiple lines
which makes the units hard to read.

Move the logic into a single shell script and call that via ExecStart.

Makefile.am

@@ -443,6 +443,12 @@ userenvgenerator_PROGRAMS = \
 	30-systemd-environment-d-generator
 endif
 
+rootlibexec_SCRIPTS = \
+	src/sulogin-shell/systemd-sulogin-shell
+
+EXTRA_DIST += \
+	src/sulogin-shell/systemd-sulogin-shell.in
+
 dist_bashcompletion_data = \
 	shell-completion/bash/busctl \
 	shell-completion/bash/journalctl \
@@ -6329,6 +6335,10 @@ src/core/%.systemd: src/core/%.systemd.in
 src/%.policy.in: src/%.policy.in.in
 	$(SED_PROCESS)
 
+src/sulogin-shell/%: src/sulogin-shell/%.in
+	$(SED_PROCESS)
+	$(AM_V_GEN)chmod +x $@
+
 shell-completion/%: shell-completion/%.in
 	$(SED_PROCESS)