Replaces code such as:
fd = open "test1.img"
ftruncate fd, size
close fd
g.add_drive "test1.img"
with the shorter and simpler:
g.add_drive_scratch size
It's simpler to use the glibc 'program_invocation_short_name(3)'
feature, and fall back to a generic solution. Also remove risky
assignments to argv[0].
This API is an easier to use version of the existing guestfs_available,
because the new API returns true/false instead of throwing an error
when a feature from the list is not available.
In truth we've had this implementation internally in the library
and several tools and in Sys::Guestfs::Lib for a long time. This
change just turns it into a publicly consumable API.
