pam: Use POSIX getpwnam_r on Solaris

pam: Use POSIX getpwnam_r on Solaris

_POSIX_PTHREAD_SEMANTICS is now always defined for Solaris, which
means we get a POSIX-conforming getpwnam_r, which takes 5 arguments.
So, add Solaris to the list of platforms that use a POSIX getpwnam_r.

Reviewed-on: http://gerrit.openafs.org/4468
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit a7d4fbd36a120b16caaddcd9d1c7f550cb14aae5)

Change-Id: I2ce885da5018b250052852cb70c70eaecd521cc5
Reviewed-on: http://gerrit.openafs.org/6289
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>