gdb: rename partial symtab expand functions of debug info readers using legacy_psymtab

As I am trying to understand the dynamic of partial_symtab::read_symtab
and partial_symtab::expand_psymtab, I think that renaming these
functions helps make it clear that they are effectively implementations
of the partial_symtab::expand_psymtab method.

gdb/ChangeLog:

	* dbxread.c (dbx_psymtab_to_symtab_1): Rename to...
	(dbx_expand_psymtab): ... this.
	(start_psymtab): Update.
	* mdebugread.c (psymtab_to_symtab_1): Rename to...
	(mdebug_expand_psymtab): ... this.
	(parse_partial_symbols): Update.
	(new_psymtab): Update.
	* xcoffread.c (xcoff_psymtab_to_symtab_1): Rename to...
	(xcoff_expand_psymtab): ... this.
	(xcoff_start_psymtab): Update.