main: error checking and adjustments for -w flag

Previously -w flag (w_width) counted both label and following separating
space. This commit makes -w flag only count the actual label width. Also
no separating space will be printed if label width is zero.

Error checking now demands that label width is non-negative.
main: open raw socket only for xping

xping-unpriv and xping-http doesn't need raw sockets and should not
have a lingering socket that is privileged. This happens if root calls
xping-unpriv or xping-http directly.

Introduce a compile time option DO_SOCK_RAW enabling the privileged
socket call. Otherwise initialize the fd and errno indicating failure
to access a the SOCK_RAW.

Makefile is adjusted to compile with DO_SOCK_RAW only for xping.
main: makefile rules with all source variables

Most of the makefile rules had two seperate lists, which were almost
the same: the sources depended on and the stuff that should be linked
together.

Make the check-* dependencies empty c files and include them in the
linking process. Then switch to all source variables ($^ and $> for
gmake and pmake respectively) for specifying objects to be linked.
On gmake $> is empty and on pmake $^ is empty, including both ensures
building works with both.
icmp-unpriv: adjust regular expressions

Older variants of iputils appear to use icmp_req instead of icmp_seq,
we want to match those as well. Also document known forms of ping output
that we want to match for future reference.