ident: replace sbuf(9) with open_memstream(3)

This change makes ident only dependant on libc functions

This makes our ident(1) more portable, also the fact that we only
depend on libc which is maintained with excellent backward compatibility
means that if one day ident is removed from base, someone using FreeBSD
22 will be able to fetch ident from FreeBSD 14 to run ident against
FreeBSD 1.0 binary

MFC After: 1 week