REORG: time: move time-keeping code and variables to clock.c

REORG: time: move time-keeping code and variables to clock.c

There is currently a problem related to time keeping. We're mixing
the functions to perform calculations with the os-dependent code
needed to retrieve and adjust the local time.

This patch extracts from time.{c,h} the parts that are solely dedicated
to time keeping. These are the "now" or "before_poll" variables for
example, as well as the various now_*() functions that make use of
gettimeofday() and clock_gettime() to retrieve the current time.

The "tv_*" functions moved there were also more appropriately renamed
to "clock_*".

Other parts used to compute stolen time are in other files, they will
have to be picked next.