printuid: fix uid_t decoding on 64-bit architectures

printuid: fix uid_t decoding on 64-bit architectures

It was not a good idea to treat uid_t as a long int type because
the latter is twice larger than uid_t on 64-bit architectures.

* defs.h (printuid): Change uid argument type from "unsigned long"
to "unsigned int".
* util.c (printuid): Likewise.  When uid equals to -1, print "-1".
* tests/uid.awk: New file.
* tests/uid.c: New file.
* tests/uid32.c: Likewise.
* tests/uid.test: New test.
* tests/uid32.test: Likewise.
* tests/Makefile.am (CHECK_PROGRAMS): Add uid and uid32.
(TESTS): Add uid.test and uid32.test.
(EXTRA_DIST): Add uid.awk.
* tests/.gitignore: Add uid and uid32.