Rearrange and tidy up code in guestfsd.c

This just tidies up the main() function in the daemon.  There is
no longer a huge fixed-sized buffer used for the kernel command
line.  Variables are moved closer to where they are used.  Some
local variables turned out to be unused - these are removed.  Finally
the part that reads the kernel command line is moved into a separate
function.

There is only minimal functional change here (it now prints out
the kernel command line).

Literal '4' replaced with 'sizeof' expression, at Jim Meyering's
suggestion during patch review.