* mlibc/include: Neutralize include guards before #include_next. Fixes compiling tinycc on Debian GNU/Linux.
* mlibc/include/string.h (ssize_t): New type. * mlibc/include/unistd.h (ssize_t): New type.
* mlibc: Rename from libc. * HACKING: Update for name change. * make/bin.make: Likewise. * make/mescc-guile.make: Likewise. * make/mescc-mes.make: Likewise. * scaffold/scaffold.make: Likewise. * make/install.make: Likewise. (install): Install mlibc into share/mlibc.