make MAIN more robust and less magic

The problem with the previous approach was that it made the parsing
of command lines dependent on the signature of each multi MAIN
candidate, making it impossible to first process @*ARGS and then
do a multi dispatch.

A different approach at fixing that would be to use the signature of
the proto of MAIN as reference instead. If somebody feels strongly
about preserving the old magic, please revert this commit and clarify
that magic command line parsing needs an explicit proto MAIN.

S06-routines.pod

@@ -3328,22 +3328,18 @@ as follows:
     # Short names
     -n                         :name
     -n=value                   :name<value>
-    -nvalue                    :name<value>     # only if not declared Bool
     -n="spacey value"          :name«'spacey value'»
     -n='spacey value'          :name«'spacey value'»
     -n=val1,'val 2',etc        :name«val1 'val 2' etc»
 
     # Long names
-    --name                     :name            # only if declared Bool
-    --name=value               :name<value>     # don't care
-    --name value               :name<value>     # only if not declared Bool
+    --name                     :name
+    --name=value               :name<value>
 
     --name="spacey value"      :name«'spacey value'»
     --name "spacey value"      :name«'spacey value'»
     --name='spacey value'      :name«'spacey value'»
-    --name 'spacey value'      :name«'spacey value'»
     --name=val1,'val 2',etc    :name«val1 'val 2' etc»
-    --name val1 'val 2' etc    :name«val1 'val 2' etc» # only if declared @
     --                                          # end named argument processing
 
     # Negation
@@ -3412,10 +3408,6 @@ then you could invoke the program with either C<-f> or C<--frompart>
 to specify the first parameter.  Likewise you could use either C<-t>
 or C<--topart> for the second parameter.
 
-If a switch of the form C<-abc> cannot be matched against any
-particular parameter, an attempt will be made to match it as if it
-had been written C<-a -b -c>.
-
 =head2 Relationship of MAIN routine with lexical setting
 
 The preceding section describes the use of C<MAIN> in the user's code.