lib: add strtol into utility library

The utility library is for functions which are shared in commands,
but that does not belong to libproc-ng.  The first function is a
wrapper for strtol that performs error checking, and exists if such
happen.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

Makefile.am

@@ -3,6 +3,7 @@ AM_CPPFLAGS = -include $(top_builddir)/config.h -I$(top_srcdir)/include
 ACLOCAL_AMFLAGS = -I m4
 SUBDIRS = \
 	include \
+	lib \
 	po \
 	proc \
 	ps \