httpservice: move sys.exit() out of serve_forever()
This is a simple refactoring to show the callers of the method, so
it's easier to reason about the impact of removing the `sys.exit()`
calls in subsequent patches.
Differential Revision: https://phab.mercurial-scm.org/D9270
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif