Move akimpersonate to libauth

Move akimpersonate to libauth

Give it its own source file and header, install the header at
depinstall time, and have aklog get the akimpersonate functionality
from libauth.

Keep the linux box copyright from aklog_main.c (but strip the trailing
whitespace), as that block was added with the akimpersonate code.

Remove all calls to afs_com_err() as is fitting for library code,
to let it build.  Do not bother removing curly braces which are
no longer needed; a future cleanup commit will catch that.