New upstream version 1.21.26.
- Use ./configure --with-default-backend=.. instead of attach-method.
- Remove Sys::Guestfs::Lib (removed upstream).
- Detect if network is available.
libguestfs_buildnet macro no longer needed.
- Enable hardened build (-fPIE, RELRO).
- Remove BRs: ncurses-devel and versioned parted.