Avoid 'head' usage in scripts that may run in initramfs

Ubuntu's busybox-initramfs package does not provide the `head` function.
Replace `head` usage with an equivalent `sed` command.

src/luks/clevis-luks-bind.in

@@ -112,7 +112,7 @@ if [ "${luks_type}" = "luks1" ]; then
     # have to wipe out the LUKSMeta slot priot to adding the new key.
     first_free_cs_slot=$(cryptsetup luksDump "${DEV}" \
                          | sed -rn 's|^Key Slot ([0-7]): DISABLED$|\1|p' \
-                         | head -n 1)
+                         | sed -n 1p)
     if [ -z "${first_free_cs_slot}" ]; then
         echo "There are no more free slots in ${DEV}!" >&2
         exit 1

src/luks/clevis-luks-common-functions

@@ -86,7 +86,7 @@ clevis_luks_read_slot() {
         local token_id
         token_id=$(cryptsetup luksDump "${DEV}" \
                     | grep -E -B1 "^\s+Keyslot:\s+${SLT}$" \
-                    | head -n 1 | sed -rn 's|^\s+([0-9]+): clevis|\1|p')
+                    | sed -n 1p | sed -rn 's|^\s+([0-9]+): clevis|\1|p')
         if [ -z "${token_id}" ]; then
             echo "Cannot load data from ${DEV} slot:${SLT}. No token found!" >&2
             return 1

src/luks/systemd/dracut/clevis/module-setup.sh.in

@@ -38,7 +38,7 @@ install() {
         /etc/services \
         @libexecdir@/clevis-luks-askpass \
         clevis-luks-common-functions \
-        head grep sed cut \
+        grep sed cut \
         clevis-decrypt \
         clevis-luks-list \
         cryptsetup \