gdb: introduce new 'maint flush ' prefix command

We currently have two flushing commands 'flushregs' and 'maint
flush-symbol-cache'.  I'm planning to add at least one more so I
thought it might be nice if we bundled these together into one place.

And so I created the 'maint flush ' command prefix.  Currently there
are two commands:

  (gdb) maint flush symbol-cache
  (gdb) maint flush register-cache

Unfortunately, even though both of the existing flush commands are
maintenance commands, I don't know how keen we about deleting existing
commands for fear of breaking things in the wild.  So, both of the
existing flush commands 'maint flush-symbol-cache' and 'flushregs' are
still around as deprecated aliases to the new commands.

I've updated the testsuite to use the new command syntax, and updated
the documentation too.

gdb/ChangeLog:

	* NEWS: Mention new commands, and that the old commands are now
	deprecated.
	* cli/cli-cmds.c (maintenanceflushlist): Define.
	* cli/cli-cmds.h (maintenanceflushlist): Declare.
	* maint.c (_initialize_maint_cmds): Initialise
	maintenanceflushlist.
	* regcache.c: Add 'cli/cli-cmds.h' include.
	(reg_flush_command): Add header comment.
	(_initialize_regcache): Create new 'maint flush register-cache'
	command, make 'flushregs' an alias.
	* symtab.c: Add 'cli/cli-cmds.h' include.
	(_initialize_symtab): Create new 'maint flush symbol-cache'
	command, make old command an alias.

gdb/doc/ChangeLog:

	* gdb.texinfo (Symbols): Document 'maint flush symbol-cache'.
	(Maintenance Commands): Document 'maint flush register-cache'.

gdb/testsuite/ChangeLog:

	* gdb.base/c-linkage-name.exp: Update to use new 'maint flush ...'
	commands.
	* gdb.base/killed-outside.exp: Likewise.
	* gdb.opt/inline-bt.exp: Likewise.
	* gdb.perf/gmonster-null-lookup.py: Likewise.
	* gdb.perf/gmonster-print-cerr.py: Likewise.
	* gdb.perf/gmonster-ptype-string.py: Likewise.
	* gdb.python/py-unwind.exp: Likewise.

gdb/cli/cli-cmds.c

@@ -151,6 +151,10 @@ struct cmd_list_element *maintenanceprintlist;
 
 struct cmd_list_element *maintenancechecklist;
 
+/* Chain containing all defined "maintenance flush" subcommands.  */
+
+struct cmd_list_element *maintenanceflushlist;
+
 struct cmd_list_element *setprintlist;
 
 struct cmd_list_element *showprintlist;