route.c: Cleanup.

Shorten and/or simplify local variable names for readability and
cleanup messy formatting.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
timer.c:age_routes() -- Refactor and cleanup duplicated threshold detection

The threshold detection for when to switch to a shortest-path-tree
from the default RP shared-tree was duplicated in three places in
the same function.  This commit factors it out and simplifies the
code into a check_spt_threshold(mrt_entry_t *) function.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
README: Update on new spt-threshold changes and feature.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>