Add new fparseln() function

Add new fparseln() function

Taken from NetBSD.

[guillem@hadrons.org:
 - Import from NetBSD instead of FreeBSD to get a 3-clause BSD license,
   instead of a 4-clause one.
 - Define compatibility macros.
 - Change library from libc to libbsd and header in man page.
 - Add copyright information to COPYING.
 - Add symbol to map file. ]

Signed-off-by: Guillem Jover <guillem@hadrons.org>