MINOR: tools: protect dladdr() against reentrant calls from the debug handler

MINOR: tools: protect dladdr() against reentrant calls from the debug handler

If a thread is currently resolving a symbol while another thread triggers
a thread dump, the current thread may enter the debug handler and call
resolve_sym_addr() again, possibly deadlocking if the underlying libc
uses locking. Let's postpone the debug signal delivery in this area
during the call. This will slow the resolution a little bit but we don't
care, it's not supposed to happen often and it must remain rock-solid.

(cherry picked from commit 2dfb63313b0d2b9d4d636dbd23688d6a20cba2cf)
Signed-off-by: Willy Tarreau <w@1wt.eu>