MFH (r272280, r272281, r272348): allow use with null user and rhost

PR:		83099 193927
MFC r272254:
Instead of creating the full range of possible ports, try to figure
out the actual number of so-called "embedded jacks" which are present
when a USB MIDI device is attaching.
MFC r272389:

Diff reduction with kernel code: instruct the compiler that the data of
these types may be unaligned to their "normal" alignment and exercise
caution when accessing them.

PR:		194071
MFC r272288,272289:

When setting environment variables in the atrun script, use the
"export foo=bar" form instead of "foo=bar; export foo" since the
former allows the shell to catch variable names that are not valid
shell identifiers.  This will cause /bin/sh to exit with an error
(which gets mailed to the at user) and it will not run the script.

Obtained from:	OpenBSD (r1.63 millert)