(gdb) set debug infrun 1
(gdb) signal SIGUSR1
Continuing with signal SIGUSR1.
infrun: clear_proceed_status_thread (process 27276)
infrun: proceed (addr=0xffffffff, signal=30, step=0)
infrun: resume (step=1, signal=30), trap_expected=1
infrun: wait_for_inferior (treat_exec_as_sigtrap=0)
infrun: target_wait (-1, status) = 27276, status->kind = stopped,
signal = SIGTRAP
infrun: infwait_normal_state
infrun: TARGET_WAITKIND_STOPPED
infrun: stop_pc = 0xeff4636
Here, it seems like telling BSD to single-step, and, to deliver
a signal at the same time, just single-steps, resulting in a SIGTRAP.