Make posix run_install cap posix-compliant

Previously the posix run_install definition used the `at` command with
the syntax "at next minute". This syntax does not work on e.g. solaris
at. On linux, "at next minute" is equivalent to "at now '+ 1min'", which
does work with solaris at. This commit modifies the posix run_install
definition to use the more compatible invocation.
Merge branch 'solaris-support'