Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Move the iconv wrapper source from libc_nonshared to libc/iconv so that
it is all in the one place again. Rename libc/iconv/iconv.c to
bsd_iconv.c. Compile the wrappers into libc.a so that WITHOUT_DYNAMICROOT
works again.

Discussed with: kib (and partly stolen from his patch)