Make use of the recently added 'getprogname' module in gnulib: replace
our guestfs_int_program_name with the getprogname() provided by the
module, since it does the same thing, and in a portable way.
As consequence of the above, use gnulib in a couple of tests that use
getprogname().
Since guestfs_int_program_name is gone, drop the configure checks
associated with it.
eea210dbf7