Move xstrtol messages into gnulib domain, when --pobase is used.

Move xstrtol messages into gnulib domain, when --pobase is used.
* lib/xstrtol.h (_STRTOL_ERROR): Move messages out of macro...
* lib/xstrtol-error.c (xstrtol_error): ...into new file.
* modules/xstrtol (Files): Distribute new file.
* m4/xstrtol.m4 (gl_XSTRTOL): Build new file.
* lib/xstrtol.c (TESTING_XSTRTO): Move tests...
* tests/test-xstrtol.c: ...into new file.
* tests/test-xstrtoul.c: Also test xstrtoul.
* tests/test-xstrtoimax.c: Also test xstrtoimax.
* tests/test-xstrtoumax.c: Also test xstrtoumax.
* tests/test-xstrtol.sh: Drive the tests.
* tests/test-xstrtoimax.sh: Likewise.
* tests/test-xstrtoumax.sh: Likewise.
* modules/xstrtol-tests: New module.
* modules/xstrtoimax-tests: Likewise.
* modules/xstrtoumax-tests: Likewise.