i-t: rewrite hooks

This produces a leaner image, doesn't fail if zdb doesn't exist,
properly handles hostnameless systems, doesn't mention crypto modules
for no reason, doesn't add useless empty executable in hopes an
eight-year-old PR is merged, uses i-t builtins for all copies

Also optimize the checkbashisms filter to spawn one (or a few) awks
instead of one per regular file and remove initramfs/hooks therefrom due
to a command -v false positive

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12017

contrib/initramfs/hooks/zfsunlock.in

@@ -1,17 +1,9 @@
 #!/bin/sh
 
-PREREQ="dropbear"
-
-prereqs() {
-    echo "$PREREQ"
-}
-
-case "$1" in
-    prereqs)
-        prereqs
-        exit 0
-    ;;
-esac
+if [ "$1" = "prereqs" ]; then
+	echo "dropbear"
+	exit
+fi
 
 . /usr/share/initramfs-tools/hook-functions