usr.sbin/ndp: Cleanup in preparation to add libxo support

• Constify rtpref_str declaration
• Remove unused h_errno declaration
• Use time_t type for expire
• Use strlcpy to set static "?" value to ifname
• Rename local variable 's' to stop shadowing global definition
• Close socket used in pfx_flush()
• Use local variables for sock() in setdefif() and getdefif()
• Increase WARNS to 3

Reviewed by: allanjude, kevans
Approved by: allanjude
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D11118