Sync with VPCS official repository:

Bugfix: using getenv+access to the real path of VPCS. (code copied from
the source code of 'which' of FreeBSD)
Patch for Debian GNU/kFreeBSD from Daniel Lintott
Flush out the output buffer while running 'echo ...'
Increase the maxinum number of tcp sessions and decrease tcp keep alive
time
Bugfix: arp table, the source ether address is not saved