*printf-posix: Fix implementation of %b directive.
* lib/vasnprintf.c (VASNPRINTF): In the %b directive implementation, fix
the precision handling, and ignore the '0' flag when a width and a
precision are both present.
* tests/test-snprintf-posix.h (test_function): Add test cases for the %x
directive and more test cases for the %b directive.
* tests/test-sprintf-posix.h (test_function): Likewise.
* tests/test-vasnprintf-posix.c (test_function): Likewise.
* tests/test-vasnwprintf-posix.c (test_function): Likewise.
* tests/test-vasprintf-posix.c (test_function): Likewise.
* modules/vasnwprintf-posix-tests (Files): Add m4/musl.m4.
(configure.ac): Invoke gl_MUSL_LIBC.