1-way delay and improvements for heterogeneous measurement

Important changes are:
e7987d1 Add 1-way delay measurement
e4f9677 Unique identifier for intermediated interval report columns
706be33 Determine unit of Kernel TCP metrics automatically
4d911fb Undo commit 517fda8 (add option -Z)
9d97a22 Use a function call to hide/show intermediated interval report columns
dc6e5fe Use a function call to change the unit of intermediated interval repo…
c5f7078 Column visibility based on involved OSes
61b4096 Enable 1-way delay measurement only when configured with --enable-debug

Minor changes are:
319c925 Variables are only used in one file. Make them static
09298ad Reduce unnecessary function call by deleting parse_help_option()
c2925f3 Functions are only needed in flowgrind.c. Make them static
caa26b1 Reduce unnecessary function calls by combining close_flow() and close…
c12241c Reduce unnecessary function calls by combining print_tcp_report_line()
0ac356e Make ASSIGN_MIN and ASSIGN_MAX robust against '++' and '--'
42d1821 Macro to determine number of parameters
cc6e74c Global variables are only used in flowgrind.c. Make them static
9de038d Make log_output() and die_if_fault_occurred() inline
7fa89ea Use enums instead of defines