Fix thinko that, with two map entries like shown below, in that order,
made autofs mix them up: the second one wasn't visible in ls(1) output,
and trying to access it would trigger mount for the first one.

foobar		host:/foobar
foo		host:/foo

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
install: re-check failed mkdir for EEXIST

Since the code stats and mkdirs in 2 separate steps, it is possible that
the directory will be created in the meantime by something else (e.g.
concurrent install).[1]

While here alter the code to properly report stat failure, previously it
would always claim it was mkdir which failed.

Noted by: royger [1]
MFC after:	1 week