net/quagga: introduce compatible installation mode

This change makes it possible to build a package not conflicting
with frr, pimd nor openbgpd introducing new port option LEGACY
enabled by default. If disabled, the package is called quagga-compat
and has following differences:

• /usr/local/bin/vtysh renamed to /usr/local/bin/qvtysh;
• libraries install to /usr/local/quagga/lib instead of /usr/local/lib;
• binaries install to /usr/local/quagga/sbin instead of /usr/local/sbin.
• manual pages install to /usr/local/quagga/man;

This is intended to ease migration from quagga to frr
making it possible to stop quagga and start frr at once
and fall back quickly if things go wrong.

Also, remove unneeded USES=ssl. Add other distfile mirrors.