The pthread_self() implementation was inconsistent with pthread_create(),
leading to corruption.  Switch to a simpler implementation using
thread-local storage.

Needed for pthread_attr_*() (and in general to make sure the stack is
a regular vma as expected by the payload).

We now have a common set of flags for C and C++, but also allow C-only flags,
which are used to:

  a) enable C99+GNU mode
  b) include the libc internal headers

although the latter could be refined even more.

Import the whole string library, as well as the ctype functions from the musl
C library.  Must is a BSD licensed library that aims to be API and mostly ABI
compatible with glibc, so it's our easiest way to provide the ABI we need
without reinventing the wheel.  I will import more of the traditional non-syscall
C library code from it and expect libc/ to eventually mostly consist of slightly
modified musl code.

Fox simplicity, the first mutex implementation will not be compatible with
PTHREAD_MUTEX_INITIALIZER, so wrap the mutex class with a structure that also
contains an intitialization flag, and initialize it on demand.

There now is a public add_object and a private set_object method, which
describe the two totally different actions much better.

Apparently glibc defines it as a macro, so it doesn't compile.  #undef it
back.

Instead of using the variable's value, we used its name.

There isn't even a name for this kind of bug.

Could be better, but waiting for vfs revamp anyway.

Also based on prex code.

Various changes to fit into the OSv environment.  This also includes
a ramfs in-memory full filesystem.  Bootfs is rewritten as a simple
unpacker into ramfs, and the fs class is modified to wrap the VFS
instead of being an abstract subclass.

Using the recently commited strncmp broke the VFS path lookup.

Unlike most glibc data types, FILE is not opaque - it is defined as _IO_FILE
in libio.h and some glibc inlines actually use it.  So we must use the same
structure.  Do this by subclassing _IO_FILE.

Unbreaks mode=release build.

as in "%016lx".