Updated some copyrights, cleaned up some header-file comments, and:

Updated some copyrights, cleaned up some header-file comments, and:

* symbols.c (symbol_create): New function, most of the guts of the old
symbol_new function.
(symbol_new): Now just checks symbol_table_frozen, calls symbol_create, and
enters the symbol into the symbol table.
* subsegs.c (section_symbol): If EMIT_SECTION_SYMBOLS is not true, and the
symbol table is frozen, call symbol_create instead of symbol_new.
* symbols.h (symbol_create, symbol_table_frozen): Declare.

* symbols.c (symbol_clear_list_pointers): Always a function now.
* struc-symbol.h (symbol_clear_list_pointers): Deleted macro version.

* symbols.c (debug_verify_symchain): New macro, defined to be
verify_symbol_chain or a cast to void, depending on DEBUG_SYMS.
(many functions): Invoke debug_verify_symchain unconditionally.