Add reentrant versions of ctime, localtime, gmtime, asctime.

Add reentrant versions of ctime, localtime, gmtime, asctime.

These cannot be implemented platform-independent, so we fall back to the native
non-reentrant versions, but lock during each access (only if ZTS is used).

To initialize/destroy the used data structures, you need to call
reentrancy_startup() before sapi_startup(), and reentrancy_shutdown() after
sapi_shutdown().