Remove dependency on bash_completion

If available, bash_completion's improvements for handling arguments
containing colons will be used, but completion will still be functional
without them.

Also unset the 'hostcomplete' shell option, as it makes it impossible
to provide completions for arguments containing an @-sign. If
bash_completion is in use, this option will already have been disabled
and replaced with something better.

zfs_completion.bash

@@ -29,6 +29,12 @@ else
     __ZPOOL_CMD="sudo zpool"
 fi
 
+# Disable bash's built-in hostname completion, as this makes it impossible to
+# provide completions containing an @-sign, which is necessary for completing
+# snapshot names. If bash_completion is in use, this will already be disabled
+# and replaced with better completions anyway.
+shopt -u hostcomplete
+
 __zfs_get_commands()
 {
     $__ZFS_CMD 2>&1 | awk '/^\t[a-z]/ {print $1}' | cut -f1 -d '|' | uniq
@@ -183,8 +189,14 @@ __zfs_complete()
 {
     local cur prev cmd cmds
     COMPREPLY=()
-    # Don't split on colon
-    _get_comp_words_by_ref -n : -c cur -p prev -w COMP_WORDS -i COMP_CWORD
+    if type _get_comp_words_by_ref &> /dev/null
+    then
+        # Don't split on colon
+        _get_comp_words_by_ref -n : -c cur -p prev -w COMP_WORDS -i COMP_CWORD
+    else
+        cur="${COMP_WORDS[COMP_CWORD]}"
+        prev="${COMP_WORDS[COMP_CWORD-1]}"
+    fi
     cmd="${COMP_WORDS[1]}"
 
     if [[ ${prev##*/} == zfs ]]
@@ -327,7 +339,10 @@ __zfs_complete()
             COMPREPLY=($(compgen -W "$(__zfs_match_explicit_snapshot) $(__zfs_list_datasets)" -- "$cur"))
             ;;
     esac
-    __ltrim_colon_completions "$cur"
+    if type __ltrim_colon_completions &> /dev/null
+    then
+        __ltrim_colon_completions "$cur"
+    fi
     return 0
 }