Add option quiet_if_down.

If a source is not connectable, ccollect.sh issues a series of error
messages such as:

$ ccollect.sh  "int 1" dummy
2009-06-25-21:04:14: ccollect 0.7.1: Beginning backup using interval int 1
[dummy] 2009-06-25-21:04:14: Beginning to backup
[dummy] ssh: connect to host Ha port 20: No route to host
[dummy] rsync: connection unexpectedly closed (0 bytes received so far) [receiver]
[dummy] rsync error: unexplained error (code 255) at io.c(600) [receiver=3.0.5]
[dummy] 2009-06-25-21:04:17: Error: source Ha:/tmp is not readable. Skipping.
2009-06-25-21:04:17: Finished

If you expect the source to be up, you want to see these messages.
However, for a notebook computer or other portable machine, it may be
normal for it to be disconnected.  If quiet_if_down is specified for
that source, then the ssh and rsync errors are suppressed and the
"Error:" prefix is removed from the "skipping" message:

$ ccollect.sh  "int 1" dummy
2009-06-25-21:03:33: ccollect 0.7.1: Beginning backup using interval int 1
[dummy] 2009-06-25-21:03:34: Beginning to backup
[dummy] 2009-06-25-21:03:37: Source Ha:/tmp is not readable. Skipping.
2009-06-25-21:03:37: Finished

I considered the alternative implementation of adding the logic to
ccollect_analyse_logs.sh to enable it to separate rsync messages
generated the initial connection test from messages generated by
rsync used for an actual backup data transfer.  Adding this approach
to ccollect.sh appeared much simpler.

ccollect.sh

@@ -285,7 +285,7 @@ while [ "${i}" -lt "${no_sources}" ]; do
    c_pre_exec="${backup}/pre_exec"
    c_post_exec="${backup}/post_exec"
    for opt in exclude verbose very_verbose rsync_options summary delete_incomplete \
-         remote_host rsync_failure_codes mtime ; do
+         remote_host rsync_failure_codes mtime quiet_if_down ; do
       if [ -f "${backup}/$opt" -o -f "${backup}/no_$opt"  ]; then
          eval c_$opt=\"${backup}/$opt\"
       else
@@ -368,7 +368,14 @@ while [ "${i}" -lt "${no_sources}" ]; do
    #
    # Verify source is up and accepting connections before deleting any old backups
    #
-   rsync "${source}" >/dev/null || _exit_err "Source ${source} is not readable. Skipping."
+   if ! rsync "${source}" >/dev/null 2>"${TMP}" ; then
+      if [ -f "${c_quiet_if_down}" ]; then
+         _exit_err "Source ${source} is not readable. Skipping."
+      else
+         cat "${TMP}"
+         _exit_err "Error: source ${source} is not readable. Skipping."
+      fi
+   fi
 
    #
    # Destination is a path

doc/ccollect.text

@@ -361,6 +361,7 @@ Additionally a source may have the following files:
    - `remote_host`         host to backup to
    - `rsync_failure_codes` list of rsync exit codes that indicate complete failure
    - `mtime`               Sort backup directories based on their modification time
+   - `quiet_if_down`       Suppress error messages if source is not connectable
 
 
 Example:
@@ -585,8 +586,8 @@ be left in the destination directory indicating failure of this backup.  If
 you have enabled delete_incomplete, then this backup will be deleted during
 the next ccollect run on the same interval.
 
-mtime
-^^^^^
+Detailed description of "mtime"
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 By default, ccollect.sh chooses the most recent backup directory for cloning or
 the oldest for deletion based on the directory's last change time (ctime).
 With this option, the sorting is done based on modification time (mtime).  With
@@ -596,7 +597,15 @@ your backups to another hard disk using cp -a or rsync -a, you should use this
 option because the ctimes are not preserved during such operations.
 
 If you have any backups in your repository made with ccollect version 0.7.1 or
-earlier, do not use this option and do not move your repository.
+earlier, do not use this option.
+
+Detailed description of "quiet_if_down"
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+By default, ccollect.sh emits a series of error messages if a source is not
+connectable.  With this option enabled, ccollect.sh still reports that the
+source is not connectable but the associated error messages generated by
+rsync or ssh are suppressed.  You may want to use this option for sources,
+like notebook PCs, that are often disconnected.
 
 Hints
 -----