gpt-auto: rename all functions that operate on a DissectedPartition object add_partit...

gpt-auto: rename all functions that operate on a DissectedPartition object add_partition_xyz()

The function for handling regular mounts based on DissectedPartition
objects is called add_partition_mount(), so let's follow this scheme for
all other functions that handle them, too. This nicely separates out the
low-level functions (which get split up args) from the high-level
functions (which get a DissectedPartition object): the latter are called
add_partition_xyz() the former just add_xyz().

This makes naming a bit more systematic. No change in behaviour.