Brought mm0 initialization up to init_execve()

Changes:
It is now possible to use do_mmap() from within mm0.

- pager_new_virtual()/delete_virtual() return addresses that are
  disjoint from find_unmapped_area() used by mmap() interface for
  anonymous or not-fixed areas.
- find_unmapped_area() now uses task->map_start task->map_end instead
  of task->start and task->end. task->start/end are still valid task
  space addresses for mmap(), but finding a new address is limited to
  map_start/map_end.

- We have both interfaces because mmap() is only useful for backed-files.
  When the pager needs to access a user memory range for example, that is
  not backed by a file and thus we need to use pager_new_virtual() instead
  of mmap() for mapping.

conts/posix/mm0/include/memfs/memfs.h

@@ -42,7 +42,7 @@
 #define MEMFS_TOTAL_SIZE		SZ_4MB
 #define MEMFS_TOTAL_INODES		128
 #define MEMFS_TOTAL_BLOCKS		2000
-#define MEMFS_FMAX_BLOCKS		40
+#define MEMFS_FMAX_BLOCKS		60
 #define MEMFS_BLOCK_SIZE		PAGE_SIZE
 #define MEMFS_MAGIC			0xB
 #define MEMFS_NAME			"memfs"