This code is shared between input.conf parser and option parser. Until
now, the performance didn't really matter. But I want to use this code
for JSON parsing too, and since JSON will have to be parsed a lot, it
should probably try to avoid realloc'ing too much.
This commit moves parsing of C-style escaped strings into a common
function, and allows using it in a way realloc can be completely
avoided, if the already allocated buffer is large enough.
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.
