DEV: dev/gdb: add fdtab dump

DEV: dev/gdb: add fdtab dump

Three functions are provided here:
  fd_dump: lists all FDs
  fd_dump_conn: lists all FDs holding a connection
  fd_dump_listener: lists all FDs holding a listener

They take no argument, and dump some of the known info. E.g. for
a connection, ctrl, xprt, flags, mux, sessions, frontend's name
and session's age are reported. Example:

  (gdb) fd_dump_conn
  fd    31: rm=0 tm=0x2 um=0 st=0x21 refc=0x1 tkov=0 gen=0 conn=0x7fffe803b600: flg=0x300 err=0 ctrl=0xdf51c0 xprt=0xdf5c80 mux=0xbaeee0 sess=0x7ffff003b570: fe=0x1e45b00 id=foo age=0ms

They are particularly slow because they iterate over all possible FDs,
so better limit them to the desired types.