Improve consistency of file commands

[turboencabulator]

• Remove a couple duplicate patterns in rm rules.
• Remove the absolute path on some /bin/rm and /bin/cp commands.
• Always use rm -f instead of rm || true or -rm (where the leading dash is Make syntax to ignore errors).
• rmdir commands were using two error suppression methods (-rmdir || true), prefer rmdir || true since it is less noisy than -rmdir.
• mkdir -p commands do not need parent directories supplied as arguments; add the -p flag if missing.

[nrnrnr]

Why remove $(MAN) here?

[turboencabulator]

It's defined as the parent directory of $(MANDIR) and $(MAN7DIR), and the -p flag will automatically create parent directories if necessary.