[EXPAND] Do not split the result of tilde expansion

A tilde expansion generates a valid pathname. Splitting it using IFS
either leaves it unchanged or changes it to something unintended.

This fixes FreeBSD sh test expansion/tilde1.0 and does not change the
outcome of the other tests.

This fixes Debian bug #601096.

Example:
  IFS=m HOME=/tmp; printf "%s\n" ~

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>