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
foreach macro for iterating through enum values

Introduction of foreach macro for iterating through enum values. Purpose
of this macro is increase the readability of the code. An example would
be:
	foreach(int *i, SOURCE, DESTINATION)
		...
Typo in doxygen comment
Fix doxygen comment of det_num_digits()
Delete empty lines
Update doxygen comments
Merge pull request #184 from alexander-zimmermann/next

Doxygen updates
Merge branch 'next' of https://github.com/alexander-zimmermann/flowgrind into foreach_loop
Unify signal handling
Make MTU, PMTU and IMTU as unsigned int
Move external variables infront of the other global variables
Merge pull request #185 from alexander-zimmermann/next

Nits
Merge pull request #183 from alexander-zimmermann/foreach_loop

foreach macro for iterating through enum values