src/types.c: Rewrite pap_addr (style)

Completely rework this function to make it much clearer.  Also rename
many of the surviving variables.

No functional change.

The following rune was helpful to me when preparing this patch:

  diff -bu <(git-show HEAD:src/types.c | perl -pe 's/\bstoreto\b/out/g; s/\bmax\b/cbyte_max/g; s/\boaddr\b/use_addr/; s/\bsalen\b/out_salen/g; s/\bstep\b/in_addrlen/g; s/\baf\b/out_af/g; s/\brrsz\b/out_rrsz/g; s/\brrty\b/in_rrty/g') src/types.c |less +/pap_addr

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>