SLL2: Translate interface indices to names on Linux only.

SLL2: Translate interface indices to names on Linux only.

print-sll.c uses HAVE_NET_IF_H, which does not always work right: the
header is in POSIX.1-2001, but the result of if_indextoname() is
irrelevant if the current OS is not Linux, in which case the packet was
captured on a different host because libpcap produces DLT_LINUX_SLL2 on
Linux only.  The result can be irrelevant on Linux too, but this does
not have an easy solution.

To reduce the problem space, switch print-sll.c to check for __linux__
instead.  In tcpdump.c print the warning about interface names only if
sll2_if_print() would print interface names.  Since HAVE_NET_IF_H has no
purpose now, remove the checks for <net/if.h>.