MFC r342576-342577,342645,342812,342881,343231,343399 (by trasz):

r342577 Make sh(1) collapse $HOME into "~" in PS1
r342576 Simplify the way we set the default sh(1) PS1
r342645 Add current working directory to the default sh prompt
r342812 Give sh(1) a proper default prompt instead of just "$".
r342881 Make sh(1) recognize the default $HOME
r343231 Don't mess with BLOCKSIZE in shell startup files
r343399 Make sh(1) support \u in PS1