gc: Rename gcalloc.d to os.d

In the future this file will hold more OS-specific functions needed by
the GC, and this file looks like a good place to put them, so all
OS-specific functions are grouped in a single module.

Also, having the prefix gc on every file, when all this is inside a gc
package, doesn't seem to make a lot of sense, thus it will be removed
progressively.
gc: Remove (de)committed memory distinction

The distinction is only useful for Windows, Posix has no standard way to
do this, even when apparently you can emulate it in Linux using
PROT_NONE while mmap()ping (but Linux don't commit memory until you
write on it, so is pointless to use it in Linux).

This distinction makes the code a little more complex, because is spread
in several parts of the GC, and it doesn't have an evident positive
impact (having memory mapped but not committed is a very transient state
anyway, except on extremely rare situations, the GC will commit all the
memory eventually, as you keep allocating, it could take more or less
time, but eventually you'll have all your memory committed just before
the first time the collector runs).
Merge pull request #496 from leandro-lucarella-sociomantic/cdgc

gc: rename module and remove (de)committed memory distinction