Fixed how "--slice" handles entries that extend earlier or later than the specified time slice.

For example, with events:

	[SUNDAY]
	0800 0930 "Event 01" SOMECATEGORY
	0930 1100 "Event 02" SOMECATEGORY

the command `agenda -sunday --slice 0830:1030` shall output:

	0830 0930 Event 01           SOMECATEGORY
	0930 1030 Event 02           SOMECATEGORY

(Indentation for clarity; disregard).
Fixed propagation of status codes from internals to the return value of `main`.