Tools/scripts/mfh: check out entire port directory even if touching only files/

• remove unused clean() function, trap does it
• fix race around mktemp/setting up rm trap
• rename filelist to dirlist so it matches its purpose
• move defaults up front to the script
• misc. minor edits

PR: 246336
Approved by: portmgr (mat@)
Differential Revision: https://reviews.freebsd.org/D24789