libflux: add 'shutdown_path' config, broker attr

Problem: the shutdown script will coexist with rc1 and rc3 as
an alternative to the "cleanup" commands currently pushed into broker
memory by rc1, but it has no path configuration.

Add builtin 'shutdown_path' config key, like to 'rc1_path' and 'rc3_path'.
Add 'broker.shutdown_path' broker attribute, like 'broker.rc1_path'
and 'broker.rc3_path'.

Update flux-config(1) and flux-broker-attributes(7).

Author: garlick

doc/man1/flux-config.rst

@@ -111,6 +111,10 @@ The following configuration keys may be printed with
    The rc1 script path used by :man1:`flux-broker`, unless overridden by
    the ``broker.rc1_path`` broker attribute.
 
+**shutdown_path**
+   The shutdown script path used by :man1:`flux-broker`, unless overridden by
+   the ``broker.shutdown_path`` broker attribute.
+
 **rc3_path**
    The rc3 script path used by :man1:`flux-broker`, unless overridden by
    the ``broker.rc1_path`` broker attribute.

doc/man7/flux-broker-attributes.rst

@@ -118,6 +118,10 @@ broker.quorum-timeout [Updates: C]
 broker.rc1_path [Updates: C]
    The path to the broker's rc1 script.  Default: ``${prefix}/etc/flux/rc1``.
 
+broker.shutdown_path [Updates: C]
+   The path to the broker's shutdown script.  Default:
+   ``${prefix}/etc/flux/shutdown``.
+
 broker.rc3_path [Updates: C]
    The path to the broker's rc3 script.  Default: ``${prefix}/etc/flux/rc3``.
 

src/common/libflux/conf.c

@@ -94,6 +94,11 @@ static struct builtin builtin_tab[] = {
         .val_installed = FLUXCONFDIR "/rc1",
         .val_intree = ABS_TOP_SRCDIR "/etc/rc1",
     },
+    {
+        .key = "shutdown_path",
+        .val_installed = FLUXCONFDIR "/shutdown",
+        .val_intree = ABS_TOP_SRCDIR "/etc/shutdown",
+    },
     {
         .key = "rc3_path",
         .val_installed = FLUXCONFDIR "/rc3",