Revert "Make mlock off stop ignoring +l 0"

This commit causes redundant "-l" mode changes to be sent out.
I will fix it differently.

This reverts commit 4ba9bf5adf7e1a0575111e430ea6fc4345e080a0.
cmode: Map mode +l 0 to mode +l 1.

Some ircds like inspircd allow a limit of 0 which allows nobody in.
This was treated as no limit by some parts of services, which is
clearly undesirable (for example, mlock -l did not revert a +l 0
change).

I think +l 0 should not be allowed so this commit changes it to +l 1
which has almost the same effect.