Move lookup_block_symbol to block.c, rename to block_lookup_symbol.

Move lookup_block_symbol to block.c, rename to block_lookup_symbol.

There is another function, lookup_symbol_aux_block, and
the names lookup_block_symbol and lookup_symbol_aux_block don't
convey any real difference between them.

The difference is that lookup_block_symbol lives in the lower level
block API, and lookup_symbol_aux_block lives in the higher level symtab API.
This patch makes this distinction clear.

gdb/ChangeLog:

	* symtab.c (lookup_block_symbol): Moved to ...
	* block.c (block_lookup_symbol):  ... here and renamed.
	All callers updated.
	* block.h (block_lookup_symbol): Declare.
	* symtab.h (lookup_block_symbol): Delete.