net/pcap: move OS-dependent code to separate files

net/pcap: move OS-dependent code to separate files

PCAP PMD queries interface information differently for Linux and
FreeBSD, OS-specific code is guarded by #ifdef. This PMD also depends on
POSIX bits, namely gettimeofday() and NAME_MAX.

Move OS-dependent code to separate files.
Replace POSIX bits with DPDK equivalents.
Rename rte_eth_pcap.c to pcap_ethdev.c, like it is in most other PMDs.

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>