Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

* fix ctm(8) to use mkstemp() instead of tempnam() for tempfile
creation.

* Tag the internal err() function with __printflike to allow checking
for non-constant format string arguments (none exist)

* Use fmtcheck() to sanitize the tar command obtained via -t to make
sure it doesn't contain extraneous format operators.

Reviewed by: mikeh
MFC after: 1 week

CTM email tools.

Reviewed by: phk
Submitted by: Stephen McKay <syssgm@devetir.qld.gov.au>