Add ICONV_{GET,SET}_ILSEQ_INVALID iconvctl.  GNU iconv returns
EILSEQ when there is an invalid character in the output codeset
while it is valid in the input.  However, POSIX requires iconv()
to perform an implementation-defined conversion on the character.
So, Citrus iconv converts such a character to a special character
which means it is invalid in the output codeset.

This is not a problem in most cases but some software like libxml2
depends on GNU's behavior to determine if a character is output
as-is or another form such as a character entity (&#NNN;).

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.