Correct a comment for %ieee754-rem-pi/2.
Import scalbn directly from fdlibm.

Since the x86 port calls this already, might as well make it available
to everyone.
Rename scalbn to fdlibm_scalbn, and use a union.
Compile in scalbn.
Fix a couple of issues in the code.

 * Use fdlibm_setexception to handle the case when the arg is NaN or
   Inf. (Previously depended on x+x doing the right thing.)
 * Fix a compiler warning about possible dangling else by adding
   braces as needed.
Add alien interface to scalbn.
Add tests for kernel:%scalbn.
Move fdlibm tests from trig.lisp to fdlibm.lisp.
Fix regression in %ENUMERATE-DIRECTORIES.

Previously (18a at least), %ENUMERATE-DIRECTORIES would return a path
even if a directory element did not exist.  This behavior is restored.

See email from cmucl-help, Sep 26, 2014.

 * src/code/filesys.lisp:
   * Fix regression.in %ENUMERATE-DIRECTORIES.  Even if the directory
     does not exist, we continue recursing instead of stopping.
 * src/general-info/release-20f.txt:
   * Update
 * tests/filesys.lisp:
   * New file adding tests for UNIX-NAMESTRING.