CA-100733: update tapdisk log when it matters.

Currently, tapdisk only writes its log file out once to avoid filling up the
filesystem with an arbitrarily big log in the case of persisting IO errors.
However, this makes it harder to see what actually/eventually happened, since
usually the 10-second watchdog causes the first and only write of the log
file, while the interesting information would be logged later. Rectify this
situation by forcing the log file updates when it's triggered by the USR1
signal handler as well as for fatal cases, since these events will not happen
continuously and will not cause the log to grow too large.