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
Use ctimenow() lib call instead of using strftime()
Update doxygen in fg_pcap
fg_pcap.c - make this long lines call more readable