Use 'unsigned char', not 'u_char'

Fixes mingw compilation for b64.c
Don't include sys/mman.h on Windows

The header doesn't exist; instead, just fall back to malloc.
Use strtok() on Windows.

See the MSDN documentation for strtok(): They don't provide a
strtok_r, but they do have a thread-safe strtok().
Merge pull request #27 from nmathewson/mingw-port

Compile correctly under mingw32, mingw64