Redefine isvalid_memblock

The redefinition of isvalid_memblock returns only
true if both fields of memblock_t are != 0.
Correct comments in header of filereader_t
Implement isenabled_testerrortimer

Implement query function isenabled_testerrortimer which returns
true if a test_errortimer_t has not been fired.
Check that sys_pagesize_vm returns power of two

Added

   assert(ispowerof2_int((uint32_t)ps)) ;

to function sys_pagesize_vm cause the coming pagecache
assumes a pagesize of a power of two.
Extend slist_t with isinlist_slist

Implemented function

   bool isinlist_slist(slist_node_t * node) ;

returns true if node is already stored in a list else false.
Extend dlist_t with isinlist_dlist

Implemented function

   bool isinlist_dlist(dlist_node_t * node) ;

which returns true if node->next != 0 else false.

Also removed sanity checks from functions and changed signature

   int   insertYYY_dlist(...) ;   // changed into
   void  insertYYY_dlist(...) ;

The same was done before with type slist_t.
Implement pagecache_t

The pagecache offers pages of size 4096 bytes no matter
of the page size of the operating system.

If bigger or smaller page sizes will be needed they can
be added later.

Currently sizes of 256/1024/4096/16384 are supported.

The added static test "use_pagecache.sh" checks that
all other modules do no more use init_vmpgage/free_vmpage.

All other modules will be converted to use pagecache after
it has been integrated into maincontext_t.
Implement in memory queue_t

The queue supports pushing and poping objects
in FIFO or LIFO order.
Rename iterator in foreach macro

With the name iter_##varname you can access
the iterator used in the forach macro.

This is useful to skip iterated nodes
if the iterator supports it.
Make unittest_time_systimer more robust

Independent timers does not synchronize exactly always.
Adding wait_systimer prevents sporadic test error.