SAFE_POPEN: Added function to safe_stdio.h

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
safe_stdio: Error messages in new format

safe_macros.c introduced a new format of error messages using the following
syntax:
<file>:<line> <function>(<parameters>) failed <optional extra message>

This patch will introduce this format to safe_stdio.c as well.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>