Fix warnings.

Two sets of warnings: one about setresgid/setresuid prototypes in util.c, the
other about use of alloca() disabling stack protection in the unit tests.

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>
(cherry picked from commit 39c89473304efea4f4e25a08bcf21a301e0d2213)
Merge pull request #62 from elly/warnings

Fix warnings; awesome - thanks for helping keep tlsdate builds clean!