Introduce a new [yet unused] function for [efficiently] getting the path to
an executable by-name without forking or using externals.

In a performance benchmark of 10,000 runs on circa 2006 hardware, f_which
out-performed `which' with an average completion time of ~2.5 seconds versus
~56 seconds.

This should be handy for future use (not that I make it a habit to call
`which' in a loop 10,000 times).
Fix to a panic found internally, bad pointer during rxeof
processing. Thanks for John Baldwin for catching this. Not
clearing the flag member of the rxbuf could result in a NULL
mbuf pointer being used.

MFC after:	2 days (this needs to get into 9.2!)