Don't abort on an unimplemented sysconf parameter. One of the documented
functions of sysconf(3) is to "test ... whether certain options are
supported", so programs are free to test for features we don't support
yet, and we're supposed to return -1 with errno set to EINVAL.

For example, Boost tested _SC_THREAD_SAFE_FUNCTIONS which we didn't
support. It would have been fine if we set EINVAL (it would switch
from *_r functions to the non-reenatrant ones) - but it wasn't fine
that we abort()ed because of this test :-)

To be on the safe side, this patch still prints a message if we see an
unknown sysconf - in case in the future we'll come across a new one we
must treat. But eventually, the message should go away too.

Implement the _SC_THREAD_SAFE_FUNCTIONS sysconf, returning 1.

_SC_THREAD_SAFE_FUNCTIONS is defined in Posix 1003.1c ("posix threads"),
and is supposed to return 1 if the thread-safe functions option is
supported (*_r() functions). Since we do implement those, we should return
1 for this sysconf.

Boost's system library uses this sysconf, and if it sees it is not
available, restorts to the _r()-less variants, for no good reason.

the debug() console function is taking a lock before it access the console driver,
it does that by acquiring a mutex which may sleep.

since we want to be able to debug (and abort) in contexts where it's not possible sleep,
such as in page_fault, a lockless debug print method is introduced.

previousely to this patch, any abort on page_fault would cause an "endless" recursive
abort() loop which hanged the system in a peculiar state.

the current code handles the case of recursive aborts incorrectly, while
the existing comment is very precise :)

abort() did the same thing as the new osv::hang(), so let's just use
osv::hang(). Note that it's important that osv::hang() doesn't print
anything - abort() does, but avoids infinite recursion that can happen
when abort()'s printing itself causes a crash, and another abort().

musl doesn't provide these defines, and it isn't really necessary to put them
in some header.

When memory is badly corrupted, our attempt to print "Aborted" in abort()
can cause an endless recursion of abort()s, filling the stack and eventually
(since we don't have a stack guard) everything. Let's just avoid printing
the "Aborted" message if inside an abort().

The current implementation returns 1, leading the JRE to optimize for
a uniprocessor host.  Unfortunately those optimizations are unsafe on SMP,
leading to hangs.

Fix by returning the real number of processors.

This is only causing confusion; change all callers to add '\n' explicitly
and drop the optional argument.

just like most libc implementations.

Of course we don't actually support locales yet, so these are trivial wrappers.

Otherwise, static constructors in functions are never called.  This
manifested itself in the jit generating wrong code when compiling a function,
as the register calling convention was not initialized, and the jvm tried
to load all method arguments into %rax.

note that as of now all functioanlity related to file or pthread locking
is not implemented or stubbed out.

confstr(NULL) is defined to return the size of required storage; use a
temporary string to make it not fault.

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.

Do nothing yet (used for CLOEXEC, meaningless for us).