netlink: add attr parser utility functions

• nlattr_get_chara() to read a string into a char array, rather than to a char *
• nlattr_get_bytes() to read an arbitrary (fixed length) byte sequence
• nlattr_get_nested_ptr() to read a nested type to a struct foo *, rather than struct foo

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42221