mutex itself was probably more costly than the PRNG itself. and
locking PRNGs is almost universally not recommended. simply add
a state parameter that the caller can hold to avoid locks.
Look it up if you really care but basically z as a format specifier
often doesn't work in mingw because reasons. Our CI was working by
accident because of meson previously ignoring the CFLAGS variable. Use
our wrapper instead for these places.
The purpose of osdep/io is to provide a wrapper for POSIX I/O functions
on Windows. The last 2 functions are utility functions which don't serve
this purpose.
Move them to a separate file.
nanahi