Clean up ASN1_item_sign_ctx() a little

Clean up ASN1_item_sign_ctx() a little

Instead of inl, outl, and outll, use in_len, out_len, and buf_out_len.
Use the appropriate types for them. Check return values properly, check
for overflow. Remove some unnecessary casts and add some for readability.
Use asn1_abs_set_unused_bits() instead of inlining it.

This removes the last direct consumer of ASN1_STRING_FLAG_BITS_LEFT
outside of asn1/a_bitstr.c. The flag is still mentioned in x509/x509_addr.c
but that will hopefully go away soon.

tweaks/ok jsing