dlopen: Do not block signals

Blocking signals causes issues with certain anti-malware solutions
which rely on an unblocked SIGSYS signal for system calls they
intercept.

This reverts commit a2e8aa0d
("Block signals during the initial part of dlopen") and adds
comments related to async signal safety to active_nodelete and
its caller.

Note that this does not make lazy binding async-signal-safe with regards
to dlopen.  It merely avoids introducing new async-signal-safety hazards
as part of the NODELETE changes.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>