lib: [strutils] general purpose string handling functions

lib: [strutils] general purpose string handling functions

This patch replaces a few functions used throughout the source:
* Renames getnum (from schedutils) to strtol_or_err
* Moves strtosize (from lib/strtosize.c)
* Moves xstrncpy (from include/xstrncpy.h)
* Adds strnlen, strnchr and strndup if not available (remove it from libmount utils)

A few Makefile.am files were modified to compile accordingly along with trivial renaming
in schedutils source code.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>