While I'm not very fond of "const", it's important for declarations
(it decides whether a symbol is emitted in a read-only or read/write
section). Fix all these cases, so we have writeable global data only
when we really need.
I find this annoying. It's the reason common/version.c exists at all.
options.c did this for the user agent, which contains the version
number. Because not including version.h means you can't build the user
agent and use it in mp_default_opts anymore, do something rather awkward
in main.c to initialize the default user agent.
