[PMTUd] rework the whole math to calculate MTU

internal changes:

• drop the concept of sec_header_size that was completely wrong and unnecessary
• bump crypto API to version 3 due to the above change
• clarify the difference between link->proto_overhead and link->status->proto_overhead. We cannot rename the status one as it would also change ABI.
• add onwire.c with documentation on the packet format and what various len(s) mean in context.
• add 3 new functions to calculate MTUs back and forth and use them around, hopefully with enough clarification on why things are done in a given way.
• heavily change thread_pmtud.c to use those new facilities.
• fix major calculation issues when using crypto (non-crypto was not affected by the problem).
• fix checks around to make sure they match the new math.
• fix padding calculation.
• add functional PMTUd crypto test this test can take several hours (12+) and should be executed on a controlled environment since it automatically changes loopback MTU to run tests.
• fix way the lowest MTU is calculated during a PMTUd run to avoid spurious double notifications.
• drop redundant checks.

user visible changes:

• Global MTU is now calculated properly when using crypto and values will be in general bigger than before due to incorrect padding calculation in the previous implementation.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>