Refactor the sample time code to make one call to gettimeofday
(aka the realtime clock in clock_gettime, when available) and
one to the monotonic clock. Stores each in more appropriately
named ProcessList fields for ready access when needed. Every
platform gets the opportunity to provide their own clock code,
and the existing Mac OS X specific code is moved below darwin
instead of in Compat.
A couple of leftover time(2) calls are converted to use these
ProcessList fields as well, instead of yet again sampling the
system clock.
Related to https://github.com/htop-dev/htop/pull/574
Code that is shared across some (but not all) platforms
is moved into a 'generic' home. Makefile.am cleanups to
match plus some minor alphabetic reordering/formatting.
As discussed in https://github.com/htop-dev/htop/pull/553
