Code review comment for lp:~jdstrand/snap-confine/ubuntu-core-launcher.nnp-off

I've addressed all of the coding issues in lp:~jdstrand/ubuntu-core-launcher/security-review-fixes/. Specifically I:
- cleaned up 'out' handling and die instead
- fixed the typo
- verify snprint() >= 512 (I increased this length) and added tests
- add test for filter missing trailing newline (I didn't change the line-too-long handling, but did add tests and verify we must end with trailing newline
- use strcmp() with "@unrestricted". add @unrestricted near miss tests
- use getresuid() instead of UBUNTU_CORE_LAUNCHER_NO_ROOT
- replace getenv() with secure_getenv() everywhere we can
- check return code of fork()
- verify return code of fclose()
- simplified final execv()

In addition:
- don't support obsoleted SNAP_APP_TMPDIR and SNAP_APP_USER_DATA_PATH
- use uid_t and gid_t instead of unsigned
- check return codes of other (f)close()s to help futureproof