ztest: take -B ./path/to/ztest, LD_LIBRARY_PATH=./path/lib:$L_L_P

This changes the behaviour of -B from the illumos one which would,
in the example in the manual, take just ./chroots/lenny;
this, however, is more versatile, and scales much better for systems
with ZFS in /usr/local, for example

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13411
Closes #1770

man/man1/ztest.1

@@ -157,7 +157,17 @@ Time per pass.
 Max loops in
 .Fn spa_freeze .
 .It Fl B , -alt-ztest Ns =
-Alternate ztest path.
+Path to alternate ("older")
+.Nm ztest
+to drive, which will be used to initialise the pool, and, a stochastic half the time, to run the tests.
+The parallel
+.Pa lib
+directory is prepended to
+.Ev LD_LIBRARY_PATH ;
+i.e. given
+.Fl B Pa ./chroots/lenny/usr/bin/ Ns Nm ,
+.Pa ./chroots/lenny/usr/lib
+will be loaded.
 .It Fl C , -vdev-class-state Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy random No (default: Sy random )
 The vdev allocation class state.
 .It Fl o , -option Ns = Ns Ar variable Ns = Ns Ar value