again taken from musl.

For now disabled:

 - floor, ceil: we have an asm implementation, but need to check if it's enough
 - various functions that mess with the floating point exception status.

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.

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.