add helpers for working with options

Add the helpers vopt_if, vopt_with and vopt_enable that simplify common
option-based operations.

Instead of a bunch of

if [ "$build_option_foo" ]; then
	configure_args+=" --with-foo"
	makedepends+=" foo-devel"
else
	configure_args+=" --without-foo"
fi

one can use

configure_args="... $(vopt_with foo)
makedepends="... $(vopt_if foo foo-devel)"

instead.

We're adding these functions to common/xbps-src/shutils/common.sh but
that might not be the ideal place. I would've preferred
common/helpers/options.sh, but helpers are only available in the
individual phases, not when the template itself gets parsed.
qemu: use new vopt helpers
emacs: use new vopt helpers

the emacs template, and previously the qemu template, were adjusted to
use the new vopt helpers, to serve as examples.

We're not going to update all other templates at once though. They'll be
adjusted once they require other changes (e.g. a version bump).