library: rework namespace calls

Functions related to namespaces were half-in half-out of the
procps library and didn't fit the standard naming scheme.

While struct { long ns[x]} is a bit clunky, its the only way
to "lock in" x. The alternative is to use ns_* variables.

This work was needed before pgrep could be converted.

proc/.gitignore

@@ -1 +1,2 @@
 test_sysinfo
+test_namespace