maint: prefer endian.h for byte order conversions

maint: prefer endian.h for byte order conversions

* bootstrap.conf (gnulib_modules): Remove byteswap. Add endian.
* src/cksum.c: Include endian.h instead of byteswap.h.
(SWAP): Remove macro.
(cksum_slice8): Use htobe32 instead of SWAP.
(output_crc): Likewise.
* src/sum.c: Include endian.h instead of byteswap.h.
(SWAP): Remove macro.
(output_bsd): Use htobe16 instead of SWAP.
(output_sysv): Use htobe16 instead of SWAP.
* .gitignore: Add /lib/endian.h.