Define PTHREAD_KEYS_MAX and _POSIX_THREAD_KEYS_MAX in a POSIX-compliant way.

Also make sysconf use PTHREAD_STACK_MIN rather than redefining its
own, different, constant.

Bug: 9997352
Change-Id: I9a8e7d2b18e691439abfb45533e82c36eee9e81d
Merge "Define PTHREAD_KEYS_MAX and _POSIX_THREAD_KEYS_MAX in a POSIX-compliant way."
Merge branch 'master' of https://android.googlesource.com/platform/bionic into jb-devel