malloc: enable assert() when malloc debugging

UCLIBC_MALLOC_DEBUGGING is useless without assert().

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
libc: add isfdtype()

isfdtype(int fd, int fdtype) check whether a file descriptor fd is
of type fdtype, where the types are defined in stat(2).  It is
supported in glibc and BSD, and used by utilities such as acpid.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>