libalias: Fix -Wcast-align compiler warnings

This fixes -Wcast-align warnings caused by the underaligned struct ip.
This also silences them in the public functions by changing the function
signature from char * to void *. This is source and binary compatible and
avoids the -Wcast-align warning.

Reviewed By: ae, gbe (manpages)
Differential Revision: https://reviews.freebsd.org/D27882