Move generic_load declaration to symfile.h

Move generic_load declaration to symfile.h

... since the implementation is in symfile.c.

At the same time, add some documentation and make sure the first
parameter's name in the declaration matches the definition.

gdb/ChangeLog:

	* defs.h (generic_load): Move from here...
	* symfile.h (generic_load): ... to here.  Rename name parameter
	to args.
	* symfile.c (generic_load): Add comment.