audit: msg.c: cleanup pack functions

This commit assumes that "transportSpefic" in header byte 0 and
"reserved" in header byte 1 are always 0.  Thus, it removes unneeded
bitmasking and while at it uses names for message types.  It also
removes redundant assignments and adds a few missing ones.

The commit simplifies all the "pack" functions; it makes them
consistent by setting all header fields in transmit order, and marking
unclean spots (i.e. correctionField assignment).

Buglets fixed in the process;
  - correctionField was not cleared in follow_up
  - pdelay_resp_f_up was using the wrong length name
  - pdelay_resp_f_up was not setting controlField and logMessageInterval
  - delay_req was not clearing flags
  - pdelay_req was not blanking the trailing bytes
  - pdelay_resp was not setting controlField and logMessageInterval
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>