mktemp() tweaks
mktemp() tweaks

libexec/modernish/sys/base/mktemp.mm:
- Detect the presence of a functioning system mktemp(1) upon
  loading the modile by actually using it to create a temporary
  file and checking the results.
- In the fallback/canonical version of mktemp() for systems without
  a native mktemp(1), when using the hardlink-back-from-a-directory
  method to safely create a temporary file directly in the main tmp
  directory, avoid the possibility of attempting a hardlink across
  filesystems by creating that directory based on the same template
  as the file to be hardlinked back to the parent directory.

modernish/modernish