unset env var when the definition is removed within the envdir (also do
not issue a warning when the file is empty)
update the test to check the envdir file removal issue
Merge branch 'kazuho/issues/rm-in-envdir'
releng for 0.16
Add --backlog option to change listen backlog from command line.

The listen backlog is currently fixed to the SOMAXCONN value, which is 128.
SOMAXCONN is sometimes not enough for heavy load servers, so this options increases
some capacity under some conditions.