lib: posix: Do not redefine PATH_MAX in unistd.h

This constant should be defined in limits.h.  Define it in limits.h in
the minimal libc, and use the definition found in newlib's includes.
Values in newlib includes range from 1024 to 4096.

The rationale is that all code should use the same value; having
buffers specified with different sizes will lead to interoperability
and out of bounds array writes.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>