WL#2737: Multi-source: Limitation on named master names

SUMMARY

In the current version of multi-source, all (C++) strings are 
allowed as names for a master.

This causes problems, since e.g. 0x0 is allowed in some
character sets.

Note also that the named master names are used to build up 
the filenames, so this causes further restrictions on what 
should be allowed names.

DESCRIPTION

1. Need to figure out a proper definition of names (A-Za-z0-9_?).
2. Need to limit the allowed names to this definition giving
   user good error message if he tries to use improper name.

NOTES
Note that all length(named_master) need to be changed.