MINOR: mux-h1: make the "show fd" helper also decode the h1s subscriber when known

MINOR: mux-h1: make the "show fd" helper also decode the h1s subscriber when known

When dumping a live h1 stream, also take the opportunity for reporting
the subscriber including the event, tasklet, handler and context. Example:

   3030 : st=0x21(R:rA W:Ra) ev=0x04(heOpi) [Lc] tmask=0x4 umask=0x0 owner=0x7f97805c1f70 iocb=0x65b847(sock_conn_iocb) back=1 cflg=0x00002300 sv=s1/recv mux=H1 ctx=0x7f97805c21b0 h1c.flg=0x80000200 .sub=1 .ibuf=0@(nil)+0/0 .obuf=0@(nil)+0/0 h1s=0x7f97805c2380 h1s.flg=0x4010 .req.state=MSG_DATA .res.state=MSG_RPBEFORE .meth=POST status=0 .cs.flg=0x00000000 .cs.data=0x7f97805c1720 .subs=0x7f97805c1748(ev=1 tl=0x7f97805c1990 tl.calls=2 tl.ctx=0x7f97805c1720 tl.fct=si_cs_io_cb) xprt=RAW

(cherry picked from commit 150c4f8b729749a074ccdde75457fbcf8c8728f0)
Signed-off-by: Willy Tarreau <w@1wt.eu>