Add docs for new --sshcopyid behavior and adjust coding style

Use "${foo}" instead of "$foo" and messages should start with uppercase
letter.

grml-debootstrap

@@ -1798,8 +1798,8 @@ iface ${interface} inet dhcp
   fi
 
   if [ -n "${SSHCOPYID}" ] ; then
-    AUTHORIZED_KEYS_SOURCE=${AUTHORIZED_KEYS_SOURCE:-$HOME/.ssh/authorized_keys}
-    AUTHORIZED_KEYS_TARGET=${AUTHORIZED_KEYS_TARGET:-$MNTPOINT/root/.ssh/}
+    AUTHORIZED_KEYS_SOURCE=${AUTHORIZED_KEYS_SOURCE:-${HOME}/.ssh/authorized_keys}
+    AUTHORIZED_KEYS_TARGET=${AUTHORIZED_KEYS_TARGET:-${MNTPOINT}/root/.ssh/}
     if ssh-add -L >/dev/null 2>&1 ; then
       einfo "Use locally available public keys to authorise root login on the target system as requested via --sshcopyid option."
       mkdir -p "${MNTPOINT}"/root/.ssh
@@ -1811,14 +1811,14 @@ iface ${interface} inet dhcp
         eend 1
         bailout 1
       fi
-    elif [ -f "$AUTHORIZED_KEYS_SOURCE" ]; then
-      einfo "copying '$AUTHORIZED_KEYS_SOURCE' to '$AUTHORIZED_KEYS_TARGET' as requested via --sshcopyid option."
-      mkdir -p "$AUTHORIZED_KEYS_TARGET"
-      chmod 0700 "$AUTHORIZED_KEYS_TARGET"
-      if cp "$AUTHORIZED_KEYS_SOURCE" "$AUTHORIZED_KEYS_TARGET" ; then
+    elif [ -f "${AUTHORIZED_KEYS_SOURCE}" ]; then
+      einfo "Copying '${AUTHORIZED_KEYS_SOURCE}' to '${AUTHORIZED_KEYS_TARGET}' as requested via --sshcopyid option."
+      mkdir -p "${AUTHORIZED_KEYS_TARGET}"
+      chmod 0700 "${AUTHORIZED_KEYS_TARGET}"
+      if cp "${AUTHORIZED_KEYS_SOURCE}" "${AUTHORIZED_KEYS_TARGET}" ; then
         eend 0
       else
-        eerror "Error: copying '$AUTHORIZED_KEYS_SOURCE' to '$AUTHORIZED_KEYS_TARGET' failed"
+        eerror "Error: copying '${AUTHORIZED_KEYS_SOURCE}' to '${AUTHORIZED_KEYS_TARGET}' failed"
         eend 1
         bailout 1
       fi

grml-debootstrap.8.txt

@@ -229,10 +229,16 @@ Options and environment variables
 *--sshcopyid*::
 
     Use locally available public keys to authorise root login on the target system.
+
     Similar to ssh-copy-id(1) (without the -i option) it checks if `ssh-add -L`
     provides any output, and if so those keys are appended to
     _/root/.ssh/authorized_keys_ (creating the file and directory (with mode 0700)).
 
+    Iff invocation of `ssh-add -L` fails and file _$HOME/.ssh/authorized_keys_ exists,
+    then it installs _$HOME/.ssh/authorized_keys_ as _/root/.ssh/authorized_keys_on
+    the target system. If a different file than _$HOME/.ssh/authorized_keys_ should
+    be used, the environment variable _AUTHORIZED_KEYS_SOURCE_ can be used.
+
 *-t*, *--target* _target_::
 
     Target partition (/dev/...) or directory (anything else without /dev at the