sh: Allow backslash-newline continuation in more places:

 * directly after a $
 * directly after ${
 * between the characters of a multi-character operator token
 * within a parameter name


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273276 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f