reactor: replace sigprocmask() with pthread_sigmask()
reactor: replace sigprocmask() with pthread_sigmask()

sigprocmask() is unspecified for multithreaded programs; pthread_sigmask()
should be used instead.  In practice they are equivalent, so this is a
cosmetic change.

cloudius-systems/seastar,scylladb/seastar,scylladb/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,sjperkins/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,sjperkins/seastar,avikivity/seastar,kjniemi/seastar,syuu1228/seastar,kjniemi/seastar,syuu1228/seastar,sjperkins/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,kjniemi/seastar