[PATCH] umask support for `hg serve`

Quick patch to allow `hg serve` to run with a umask specified in the config.

Adds a couple lines of code at the top of the serve command to put it
into a umask if one has been defined. Also added a small doc blurb for it.

The config section I put the option in is web, although I am not fully
certain that is the best place for it.

hg-umask.patch
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