Problem: When using ch_read() with zero timeout, can't tell the difference
between reading an empty line and nothing available.
Solution: Add ch_canread().
Problem: An exiting job is detected with a large latency.
Solution: Check for pending job more often. (Ozaki Kiichi) Change the
double loop in mch_inchar() into one.
Problem: It is not possible to use plugins in an "after" directory to tune
the behavior of a package.
Solution: First load plugins from non-after directories, then packages and
finally plugins in after directories.
Reset 'loadplugins' before executing --cmd arguments.
