psi_file: psi_free=>psi_close

Help clarify some of the code.
psi_file: Add psi_read method

Also update file sanity test to read and print out the first 15 bytes
of the given file.
sanity: Close child stdout, suppress log messages

This way the sanity testing can be done without having to dig
through any stdout messages the test program sends. stderr is left
open, because we do want to know the specifics of any error
encountered during execution of a test.
Merge branch 'sanity-test-suite'