do not parse even header bits on short packets

wasted time / invalid debug messages
stdlib allocation wrappers

My philosophy on this issue has evolved over time.
The previous lack of general error-checking on allocation
routines was intentional, but now I think we should check.
Add allocation error checks in libdmn

There's only 3 of them anyways.
switch allocations to x-funcs
use separate buffers for recvmmsg
Remove refs to systemd login lib

This should have been done in 19cc1d06, it's
just leftover cruft.
Clean up #include style

<> should be the norm for system dirs and -I dirs
"" should be for same-dir local, uninstalled includes