If the name of an event flag clashes with an already defined C identifier,
we will have a problem anyway. Besides, variable names aren't mangeled with
a prefix either. Also, in the next version (with pv types) event flags will
most probably be a part of the global variable struct and thus also not
mangled. Lastly, it seems a bad idea to make escaped C code depend on the
naming convention.

Thanks to Andrew Johnson who reminded me that identifiers that start with
"_" are reserved for the (C/C++) implementation, according to the standard.

The new boolean member multi_pv of struct func_symbol represents whether
multi-pv arguments are allowed by default, i.e with -p in effect. The code
generator takes this into account when deciding what to do for a multi-PV
argument. The behaviour is now exactly as described in the release notes.