bump_revision.pl: minor usability improvements

• use env -S to split the shebang line (RhToad@#bsdports)
• launder -p PORTSDIR argument and chdir to it so it doesn't matter which cwd bump_revision.pl is executed from
• if INDEX not found, hint the user that also his -p option might be off, because the -i INDEX option defaults to a file relative to -p PORTSDIR.