Add AT_EACCESS to Linux, Solaris, and Fuchsia.

Add AT_SYMLINK_FOLLOW, AT_REMOVEDIR, and  _AT_TRIGGER to Solaris.

Constants for the linux nfqueue netlink protocol (userspace firewall).

Continuation of #1562 and #1571.

fix #1633

They are missing in the CI’s linux-api-headers, and not widely available
yet either.

memmem is a non-standard extension, first added in GNU libc and later
ported to other systems. Support for it is non-uniform, thus it was only
added to platforms that seem to support it.

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

Only Netlink interface link attributes up IFLA_STATS are currently
available. This commit adds IFLA_COST to IFLA_PROTO_DOWN.

See https://github.com/torvalds/linux/blob/54ecb8f7/include/uapi/linux/if_link.h#L106