gimp_spawn_async() is similar to, but more limited than,
g_spawn_sync().  Unlike the latter, gimp_spawn_async() uses
vfork(), instead of fork(), when possible.

On Linux, a process that uses large amounts of memory (as GIMP may)
can hang during a fork() if overcommitting is enabled, and there's
not enough memroy.  Using vfork() avoids that, since it doesn't
duplicate the parent's address space.