server: Deprecate ls -p.

In v0.6.0 the -p option will have the same meaning as for the rm and
touch commands: perform pathname match. Also the default for the ls
command will be changed to list full paths while the new -b option
must be given to print only the basename (i.e., the current behaviour
if -p is not given).

This commit is a preparation for these incompatible changes which
does not break existing scripts. It does the following

* The -p option is deprecated in favor of its synonym, the new -F
option (for full path).

* The new -b option instructs the command to perform basename matching
and print only the basename of the matching files. It is currently
a no-op.

With the patch applied, scripts are supposed to specify either -b or
-F as appropriate.  In v0.6.0 the semantics of -p will be changed as
described above, the default will be changed to print the full path,
and -F will be deprecated because it is a no-op then.