ospfd: Some small tweaks to the SPF execution reason patch

ospfd: Some small tweaks to the SPF execution reason patch

* ospf_spf.h: use an enum for the reason, and have it as a new argument to
  ospf_spf_calculate_schedule, no need for additional call, and let compiler
  do the checking.
* ospf_spf.c: format changes - Quagga coding style places function names
  at the start of a new line, for easy grepping for definition.
  (ospf_spf_calculate_timer) Change the log format of SPF execution time to
  avoid ginormous line, and make logging conditional, as is the norm.

(cherry picked from commit b6eef003e1a79471addea0b01853b08aed812cc8)

Conflicts:
	ospfd/ospf_spf.c