Change full path subcommand flag from -p to -P

Commit d2f3e29 introduced the -p option which outputs full paths
for vdevs to multiple zpool subcommands. When this was merged
there was no conflict for this flag letter. However it's certain
there will be a conflict with the -p (parsable) flag used by other
subcommands. Therefore, -p is being changed to -P to avoid this.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4368