pmap: Fix output format of VmFlags.

pmap: Fix output format of VmFlags.

In the headers, the space was misplaced; for example, "pmap -XX $$"
outputs "VmFlagsMapping" (without a space). Use justify_print() instead
of printf().

There was also an extra space in the output, because vmflags[] (from the
"VmFlags:" line) always ends with a space. Overwriting this last space
with a null byte fixes this misalignment.