ada: Fix inconsistencies in conversion functions from Duration

ada: Fix inconsistencies in conversion functions from Duration

The 3 units Ada.Calendar, GNAT.Calendar and GNAT.Sockets contain conversion
functions from the Duration fixed-point type that implement the same idiom
but with some inconsistencies:

  * GNAT.Sockets only handles Timeval_Duration, i.e. positive Duration, and
    is satisfactory, although a simpler implementation can be written,

  * GNAT.Calendar mishandles negative Duration values, as well as integral
    Duration values,

  * Ada.Calendar mishandles negative Duration values, and rounds nanoseconds
    instead of truncating them.

gcc/ada/ChangeLog:

	* libgnat/a-calend.adb (To_Struct_Timespec_64): Deal with negative
	Duration values and truncate the nanoseconds too.
	* libgnat/g-calend.adb (timeval_to_duration): Unsuppress overflow
	checks.
	(duration_to_timeval): Likewise.  Deal with negative Duration values
	as well as integral Duration values.
	* libgnat/g-socket.adb (To_Timeval): Simplify the implementation.