library: eliminate inappropriate '__BEGIN_DECLS' macro

This patch simply eliminates that glibc specific macro
from all header files which contain no public callable
functions. After all, if user code can't link to them,
then protection from C++ name mangling is unnecessary.

[ we also remove any related '#include <features.h>' ]

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/readproc.h

@@ -19,8 +19,6 @@
 // the following is development only, forcing display of "[ duplicate ENUM ]" strings
 // #define FALSE_THREADS        /* set most child string fields to NULL */
 
-__BEGIN_DECLS
-
 
 // This is to help document a transition from pid to tgid/tid caused
 // by the introduction of thread support. It is used in cases where
@@ -261,5 +259,4 @@ proc_t* readeither(PROCTAB *__restrict const PT, proc_t *__restrict x);
 int look_up_our_self(proc_t *p);
 void closeproc(PROCTAB* PT);
 
-__END_DECLS
 #endif