dpv(3): MFC r330943, r335264

r330943:
Fix bad error messages from dpv(3)

Before = dpv: <func>: posix_spawnp(3): No such file or directory
After = dpv: <path/cmd>: No such file or directory

Most notably, show the 2nd argument being passed to posix_spawnp(3)
so we know what path/cmd failed.

Also, we don't need to have "posix_spawnp(3)" in the error message
nor the function because that can [a] change and [b] traversed using
a debugger if necessary.

r335264:
Fix comparison between pointer and char literal

PR: misc/204252
Reported by: David Binderman <dcb314@hotmail.com>
Sponsored by: Smule, Inc.