Parse a `user:group' specifier (e.g. the first argument of chown utility).
Files:
-lib/inttostr.h
-lib/intprops.h
lib/userspec.c
lib/userspec.h
m4/userspec.m4
xalloc
xstrtol
strdup
-gettext
+gettext-h
+intprops
+inttostr
+stdbool
configure.ac:
gl_USERSPEC