MFC r350858-r350859, r350987, r351170

r350858:
ping6: Add missing static keyword for a global variable

This fixes -Wmissing-variable-declarations error when compiled with
WARNS=6.

Submitted by: Ján Sučan <sucanjan@gmail.com>
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21214

r350859:
ping6: Remove unnecessary level of indirection from dnsdecode() parameter

The `sp' pointer doesn't need to be modified in the caller of
dnsdecode().

This fixes -Wcast-qual error (`must have all intermediate pointers
const qualified to be safe') when compiled with WARNS=6.

Submitted by: Ján Sučan <sucanjan@gmail.com>
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21215

r350987:
ping6: Fix data type of a variable for a packet sequence number

Submitted by: Ján Sučan <sucanjan@gmail.com>
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21218

r351170:
ping6: Fix dnsdecode() bug introduced by r350859

Revision 350859 removed level of indirection that was needed for setting the
caller's `cp' pointer. dnsdecode() uses return value to indicate error or
success. It returns pointer to a buffer holding a decompressed DNS name or
NULL. The caller uses that value only to find out the result, not for accessing
the buffer.

We use the return value to propagate the new value of `cp' pointer to
the caller instead of using an output argument.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC-With: 350859
Sponsored by: Google, Inc (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21266