gdb: Add a macro to walk memory allocations

gdb: Add a macro to walk memory allocations

Add a macro to walk all memory allocations in a core file.

load the file in gdb:
source /path/to/frr/gdb/lib.txt
mg_walk mg_first
(gdb) mq_walk mg_first
showing active allocations in memory group libfrr
memstats: BFD info:0
memstats: Buffer:5
memstats: Buffer data:0
memstats: Host config:6
memstats: Completion item:0
memstats: Command Tokens:2585
memstats: Command Token Text:1951
memstats: Command Token Help:1951
memstats: Command Argument:0
memstats: Command Argument Name:575
memstats: Command Match Stack:0
memstats: Lexer token (temporary):0
memstats: Path Computation Algorithms:0
memstats: Distribute ctx:0
memstats: Distribute list:0
memstats: Dist-list ifname:0
memstats: Dist-list name:0
memstats: error information:0
memstats: Access List:0
memstats: Access List Str:0
memstats: Access Filter:0
memstats: RCU thread:0
<snip>

Signed-off-by: Donald Sharp <sharpd@nvidia.com>