Replace PARA_VSNPRINTF by xvasprintf().

Replace PARA_VSNPRINTF by xvasprintf().

The PARA_VSNPRINTF macro is rather clumsy, and too large to be inlined.
Moreover, it does not return the length of the formated string, so
users have to call strlen() after the call to PARA_VSNPRINTF(). This
is extra work which can easily be avoided since the number of bytes
written is returned by the underlying call to vsnprintf().

This patch replaces the macro by the public function xvasprintf(),
which is similar to the non-standard vasprintf() on GNU systems. It
also adds xasprintf(), a similar variant which takes a variable number
of arguments.

Unlike PARA_VSNPRINTF, xasprintf() and xvasprintf() return the number
of bytes written. This relies on vsnprintf() conforming to the C99
standard and breaks in particular on glibc 2.0 systems. Since glibc
2.0 is about 15 years old, this is unlikely to cause problems on
real systems.

All users which called strlen() right after xvasprintf() are changed
to use the return value of xvasprintf() instead.