implement more io_uring opcodes

[vrischmann]

This adds more io_uring opcodes.

The following are available since kernel 5.11:

• shutdown (equivalent to this from liburing)
• renameat (equivalent to this from liburing)
• unlinkat (equivalent to this from liburing)

The following are available since kernel 5.15:

• mkdirat (equivalent to this from liburing)
• symlinkat (equivalent to this from liburing)
• linkat (equivalent to this from liburing)

[andrewrk]

Thanks!