Import iocage basejails as Bastille thin jails

I think these are the same concept, but with slightly different execution.
The main idea is to have a central base system that is shared (readonly) among
multiple jails. When this base system is updated, all the jails that reference
it immediately see the new system files.

This is implemented in iocage as a set of individual zfs mounts, one per
system directory. In Bastille, each system directory is symlinked into a
subdirectory of a single zfs mount.

My test plan here was to import an exported iocage basejail and verify that
its Bastille version has the right fstab and symlink changes:
```
Validating file: ssl_2021-11-19.zip...
File validation successful!
Importing 'ssl' from foreign compressed .zip archive.
Archive:  ssl_2021-11-19.zip
 extracting: ssl_2021-11-19
 extracting: ssl_2021-11-19_root
Receiving ZFS data stream...
Generating jail.conf...
Updating symlinks...
ln: usr/src: Directory not empty
Warning: directory usr/src on imported jail was not empty and will not be updated by Bastille
Container 'ssl' imported successfully.
```

usr/local/share/bastille/import.sh

@@ -181,15 +181,21 @@ generate_config() {
         if [ -n "${JSON_CONFIG}" ]; then
             IPV4_CONFIG=$(grep -wo '\"ip4_addr\": \".*\"' "${JSON_CONFIG}" | tr -d '" ' | sed 's/ip4_addr://')
             IPV6_CONFIG=$(grep -wo '\"ip6_addr\": \".*\"' "${JSON_CONFIG}" | tr -d '" ' | sed 's/ip6_addr://')
-	    DEVFS_RULESET=$(grep -wo '\"devfs_ruleset\": \".*\"' "${JSON_CONFIG}" | tr -d '" ' | sed 's/devfs_ruleset://')
-	    DEVFS_RULESET=${DEVFS_RULESET:-4}
+            DEVFS_RULESET=$(grep -wo '\"devfs_ruleset\": \".*\"' "${JSON_CONFIG}" | tr -d '" ' | sed 's/devfs_ruleset://')
+            DEVFS_RULESET=${DEVFS_RULESET:-4}
+            IS_THIN_JAIL=$(grep -wo '\"basejail\": .*' "${JSON_CONFIG}" | tr -d '" ,' | sed 's/basejail://')
+            CONFIG_RELEASE=$(grep -wo '\"release\": \".*\"' "${JSON_CONFIG}" | tr -d '" ' | sed 's/release://' | sed 's/\-[pP].*//')
+            ALLOW_EMPTY_DIRS_TO_BE_SYMLINKED=1
         fi
     elif [ "${FILE_EXT}" = ".tar.gz" ]; then
         # Gather some bits from foreign/ezjail config files
         PROP_CONFIG="${bastille_jailsdir}/${TARGET_TRIM}/prop.ezjail-${FILE_TRIM}-*"
         if [ -n "${PROP_CONFIG}" ]; then
             IPVX_CONFIG=$(grep -wo "jail_${TARGET_TRIM}_ip=.*" ${PROP_CONFIG} | tr -d '" ' | sed "s/jail_${TARGET_TRIM}_ip=//")
+            CONFIG_RELEASE=$(echo ${PROP_CONFIG} | grep -o '[0-9]\{2\}\.[0-9]_RELEASE' | sed 's/_/-/g')
         fi
+        # Always assume it's thin for ezjail
+        IS_THIN_JAIL=1
     fi
 
     # If there are multiple IP/NIC let the user configure network
@@ -238,8 +244,7 @@ generate_config() {
         warn "Warning: See 'bastille edit ${TARGET_TRIM} jail.conf' for manual network configuration."
     fi
 
-    if [ "${FILE_EXT}" = ".tar.gz" ]; then
-        CONFIG_RELEASE=$(echo ${PROP_CONFIG} | grep -o '[0-9]\{2\}\.[0-9]_RELEASE' | sed 's/_/-/g')
+    if [ "${IS_THIN_JAIL:-0}" = "1" ]; then
         if [ -z "${CONFIG_RELEASE}" ]; then
             # Fallback to host version
             CONFIG_RELEASE=$(freebsd-version | sed 's/\-[pP].*//')
@@ -337,6 +342,13 @@ update_symlinks() {
     for _link in ${SYMLINKS}; do
         if [ -L "${_link}" ]; then
             ln -sf /.bastille/${_link} ${_link}
+        elif [ "${ALLOW_EMPTY_DIRS_TO_BE_SYMLINKED:-0}" = "1" -a -d "${_link}" ]; then
+            # -F will enforce that the directory is empty and replaced by the symlink
+            ln -sfF /.bastille/${_link} ${_link} || EXIT_CODE=$?
+            if [ "${EXIT_CODE:-0}" != "0" ]; then
+              # Assume that the failure was due to the directory not being empty and explain the problem in friendlier terms
+              warn "Warning: directory ${_link} on imported jail was not empty and will not be updated by Bastille"
+            fi
         fi
     done
 }