[DCCP] tfrc: Identify TFRC table limits and simplify code

This
 * adds documentation about the lowest resolution that is possible within
   the bounds of the current lookup table
 * defines a constant TFRC_SMALLEST_P which defines this resolution
 * issues a warning if a given value of p is below resolution
 * combines two previously adjacent if-blocks of nearly identical
   structure into one

This patch does not change the algorithm as such.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>