replace strerror() usage with threadsafe "%m" format string

replace strerror() usage with threadsafe "%m" format string

strerror() is not threadsafe.  It uses a buffer to build messages of the form
"Unknown error 387689".

syslog() provides a %m format which is equivalent to strerror(errno).
As a GNU extension, this is also accepted by printf and friends.
At least in the current implementation, it is correctly threadsafe.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>