Previously we had two different mutex types - "mutex_t" defined by
<osv/mutex.h> for use in C code, and "mutex" defined by <mutex.hh>
for use in C++ code. This is difference is unnecessary, and causes
a mess for functions that need to accept either type, so they work
for both C++ and C code (e.g., consider condvar_wait()).

So after this commit, we have just one include file, <osv/mutex.h>
which works both in C and C++ code. This results in the same type
and same functions being defined, plus some additional conveniences
when in C++, such as method variants of the functions (e.g.,
m.lock() in addition to mutex_lock(m)), and the "with_lock" function.

The mutex type is now called either "mutex_t" or "struct mutex" in
C code, or can also be called just "mutex" in C++ code (all three
names refer to an identical type - there's no longer a different
mutex_t and mutex type).

This commit also modifies all the includers of <mutex.hh> to use
<osv/mutex.h>, and fixes a few miscelleneous compilation issues
that were discovered in the process.