* posix/sys/types.h: Define type useconds_t if not done already.
* posix/unistd.h: Likewise. Use this in ualarm and usleep
declarations.
* sysdeps/generic/bits/types.h: Define type __useconds_t.
* sysdeps/unix/sysv/linux/alpha/bits/types.h: Likewise.
* sysdeps/unix/sysv/linux/bits/types.h: Likewise.
* sysdeps/unix/sysv/linux/mips/bits/types.h: Likewise.
* sysdeps/unix/sysv/linux/sparc/bits/types.h: Likewise.
* sysdeps/unix/sysv/sysv4/solaris2/bits/types.h: Likewise.