New API function (GC_dump_named) to produce named dumps

New API function (GC_dump_named) to produce named dumps

gc_dump() now prints a label (name) for the dump, by default the name
is created using the current garbage collection number (GC_get_gc_no).
An arbitrary name could be provided using GC_dump_named() instead.
The naming makes it easier to work with multiple dumps in a single log.

* include/gc.h (GC_dump_named): New public API function declaration.
* include/gc.h (GC_dump): Update comment (the current collection number
is printed at the beginning of the dump).
* misc.c [!NO_DEBUGGING] (GC_dump): Just call GC_dump_named(NULL).
* misc.c [!NO_DEBUGGING] (GC_dump_named): Move code from GC_dump;
start dump with the "GC Dump" followed by either the name specified or
the current collection number (if name is null).