Make symfile_add_flags and objfile->flags strongly typed

This makes these flag types be "enum flag" types.  The benefit is
making use of C++'s stronger typing -- mixing the flags types by
mistake errors at compile time.

This caught one old bug in symbol_file_add_main_1 already, fixed by
this patch as well:

  @@ -1318,7 +1326,7 @@ symbol_file_add_main_1 (const char *args, int from_tty, int flags)
	what is frameless.  */
     reinit_frame_cache ();

  -  if ((flags & SYMFILE_NO_READ) == 0)
  +  if ((add_flags & SYMFILE_NO_READ) == 0)
       set_initial_language ();
   }

Above, "flags" are objfile flags, not symfile_add_flags.  So that was
actually checking for "flag & OBJF_PSYMTABS_READ", which has the same
value as SYMFILE_NO_READ...

I moved the flags definitions to separate files to break circular
dependencies.

Built with --enable-targets=all and tested on x86-64 Fedora 23.

gdb/ChangeLog:
2016-10-26  Pedro Alves  <palves@redhat.com>

	* coffread.c (coff_symfile_read): Use symfile_add_flags.
	* dbxread.c (dbx_symfile_read): Ditto.
	* elfread.c (elf_symfile_read): Ditto.
	* inferior.h: Include symfile-add-flags.h.
	(struct inferior) <symfile_flags>: Now symfile_add_flags.
	* machoread.c (macho_add_oso_symfile, macho_symfile_read_all_oso)
	(macho_symfile_read, mipscoff_symfile_read): Use
	symfile_add_flags.
	* objfile-flags.h: New file.
	* objfiles.c (allocate_objfile): Use objfile_flags.
	* objfiles.h: Include objfile-flags.h.
	(struct objfile) <flags>: Now an objfile_flags.
	(OBJF_REORDERED, OBJF_SHARED, OBJF_READNOW, OBJF_USERLOADED)
	(OBJF_PSYMTABS_READ, OBJF_MAINLINE, OBJF_NOT_FILENAME): Delete.
	Converted to an enum-flags in objfile-flags.h.
	(allocate_objfile): Use objfile_flags.
	* python/py-objfile.c (objfpy_add_separate_debug_file): Remove
	unnecessary local.
	* solib.c (solib_read_symbols, solib_add)
	(reload_shared_libraries_1): Use symfile_add_flags.
	* solib.h: Include "symfile-add-flags.h".
	(solib_read_symbols): Use symfile_add_flags.
	* symfile-add-flags.h: New file.
	* symfile-debug.c (debug_sym_read): Use symfile_add_flags.
	* symfile-mem.c (symbol_file_add_from_memory): Use
	symfile_add_flags.
	* symfile.c (read_symbols, syms_from_objfile_1)
	(syms_from_objfile, finish_new_objfile): Use symfile_add_flags.
	(symbol_file_add_with_addrs): Use symfile_add_flags and
	objfile_flags.
	(symbol_file_add_separate): Use symfile_add_flags.
	(symbol_file_add_from_bfd, symbol_file_add): Use symfile_add_flags
	and objfile_flags.
	(symbol_file_add_main_1): : Use objfile_flags.  Fix add_flags vs
	flags confusion.
	(symbol_file_command): Use objfile_flags.
	(add_symbol_file_command): Use symfile_add_flags and
	objfile_flags.
	(clear_symtab_users): Use symfile_add_flags.
	* symfile.h: Include "symfile-add-flags.h" and "objfile-flags.h".
	(struct sym_fns) <sym_read>: Use symfile_add_flags.
	(clear_symtab_users): Use symfile_add_flags.
	(enum symfile_add_flags): Delete, moved to symfile-add-flags.h and
	converted to enum-flags.
	(symbol_file_add, symbol_file_add_from_bfd)
	(symbol_file_add_separate): Use symfile_add_flags.
	* xcoffread.c (xcoff_initial_scan): Use symfile_add_flags.

gdb/solib.h

@@ -26,6 +26,8 @@ struct target_ops;
 struct target_so_ops;
 struct program_space;
 
+#include "symfile-add-flags.h"
+
 /* Called when we free all symtabs, to free the shared library information
    as well.  */
 
@@ -34,7 +36,7 @@ extern void clear_solib (void);
 /* Called to add symbols from a shared library to gdb's symbol table.  */
 
 extern void solib_add (const char *, int, struct target_ops *, int);
-extern int solib_read_symbols (struct so_list *, int);
+extern int solib_read_symbols (struct so_list *, symfile_add_flags);
 
 /* Function to be called when the inferior starts up, to discover the
    names of shared libraries that are dynamically linked, the base