MFC r331641, r331644, r332158

r331641:
Fix access to ifru_buffer on freebsd32.

Make all kernel accesses to ifru_buffer go via access functions
which take the process ABI into account and use an appropriate union
to access members in the correct place in struct ifreq.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14846

r331644:
Fix a whitespace bug missed in refactoring prior to r331641.

MFC with: r331641

r332158:
Remove the thread argument from ifr_buffer_*() accessors.

They are always used in a context where curthread is the correct thread.
This makes them more similar to the ifr_data_get_ptr() accessor.