Some reentrancy changes

[J. Johnston]

I have checked in some changes to strtok, mblen, mbtowc, and wctomb.  These
functions were declaring a static area to use with their reentrant equivalents.
I have changed them to use areas in the default reentrancy structure.  In addition,
the libc/include/sys/reent.h has been modified to add macros for initializing
a reentrant structure that has been dynamically allocated.

-- Jeff J.