Assume <stdint.h> and fixed-width types

Assume <stdint.h> and fixed-width types

Make unconditional use of <stdint.h> and fixed-width types such as
uint32_t.  k5-plugin.h doesn't use any special integer types, so
remove the conditional include block there.  Nothing uses
INT64_FMT/UINT64_FMT, so leave those out of k5-platform.h for now.