sysutils/grub2: Fix wrong lz4 endianness and general port cleanup

Due to lack of inclusion of <sys/endian.h>, the lz4 code incorrectly
assumes a big-endian system. The result issues manifest with errors like,
"error: no such device: <pool id>." and "lz3 decompression failed" at the
grub prompt. Modify existing patch to add <sys/endian.h>.

While here, simplify the port with OPTIONS_SUB framework and fix the
manpage stuff on the options which apparently has been broken since
this unmaintained port was staged.

PR: 192066
Submitted by: Andrey Zholos