Exists

Implements the function exists, which can be called on a given path
name.  If the given path does not exist, will return false.  If an error
was encountered, will inform the user of the error.  If both of these
cases are false, will return true (for the file exists).

This function is implemented in runtime code, in the file chpl-file-utils.c
and is called through a module interface found in FileSystem.chpl.
I added the test test/io/lydia/exists/fileOrDirExists.chpl, which tests
correct behavior when the file does not exist, when a file or directory
exists, and that a link to either passes.
Following up on this, since chpl_fs_exists() always sets the returned err, the initialization of syserr is actually superfluous, though not harmful. But maybe an additional thing that needs doing is to document that chpl_fs_exists() produces err=ENOERR when there is no error.