inpcb: Make some cosmetic improvements to in_pcbbind()

• Use the local var "laddr" instead of sin->sin_addr in one block.
• Use in_nullhost() instead of explicit comparisons with INADDR_ANY.
• Combine multiple socket options checks into one.
• Fix indentation.
• Remove some unhelpful comments.

This is in preparation for some simplification and bug-fixing.

No functional change intended.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D47451