mail/fetchmail: Fix shell's 'Bad -c option' in rcscript.

Turns out that our fetchmail_dump_config() function needs to add
one more level of quoting because it's being unquoted and word split
twice, once by su's shell, and again by sh.

While here, change sh to /bin/sh to make the intention clearer.

Bump PORTREVISION to get the fix out onto the systems.

PR: 250691
Reported by: Helmut Ritter <freebsd-ports@charlieroot.de>
Approved by: chalpin@cs.wisc.edu
MFH: 2020Q4 (blanket, one-line tested working fix, 4-eyes principle)