Fix path when user provides tshark_path using LiveCapture.

[miaotony]

When I provided a tshark_path parameter like D:\\Program Files\\Wireshark\\tshark.exe, using LiveCapture called tshark in Dumpcap subprocess, and nothing captured.

Therefore I tried to run it on a Linux machine which tshark was in the default path, the program captured successfully.
And you can see that the program in Dumpcap subprocess is dumpcap.

After carefully reviewing the source code, I find that when user provides his own tshark_path, the process_name will not be spliced to path, which is the bug of this problem.

The commit 85adafa did not solve this bug, so in this PR I try to fix it.

A user can provide a path like D:\\Program Files\\Wireshark\\tshark.exe or D:\\Program Files\\Wireshark, and process_name will be joined. It is also feasible for Linux.