Ticket #3805: fix --help option to show correct syntax for editor, viewer and mcdiff

Ticket #3805: fix --help option to show correct syntax for editor, viewer and mcdiff

Before the patch: --help shows the same text for all tools, only "mc"
is replaced by tool name. For example, "mcedit --help" says:

        Usage:
          mcedit [OPTION...] [+number] [this_dir] [other_panel_dir]
         +number - Set initial line number for the internal editor

which is wrong: mcedit does not take [this_dir] [other_panel_dir]
syntax, that's mc syntax.

After the patch, each tool has its own syntax string shown.
"Usage:" message for each tool is as follows now:

  mc [OPTION...] [this_dir] [other_panel_dir]

  mcedit [OPTION...] [+lineno] file1[:lineno] [file2[:lineno]...]

  mcview [OPTION...] file

  mcdiff [OPTION...] file1 file2

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>