Return current wall-clock time as null-terminated string

Enhance fg_time.[c|h] by two functions to the current wall-clock time as
null-terminated string
Use fg_time.[c|h] instead of calling strftime() directly

This patch fixes issue #64
Merge remote-tracking branch 'upstream/next' into fix-issue-64
Merge remote-tracking branch 'origin/next' into fix-issue-64
Merge remote-tracking branch 'origin/next' into fix-issue-64

Conflicts:
	src/fg_time.h
Merge remote-tracking branch 'origin/next' into fix-issue-64
Merge remote-tracking branch 'origin/next' into fix-issue-64
Merge pull request #171 from alexander-zimmermann/fix-issue-64

Use fg_time.[c|h] instead of calling strftime() directly