[sieve] added varlist.{c,h} in anticipation of the variables extension

Defines a named linked-list of strarray_t's.  Each variable is stored as
a strarray_t in var, with the variable name stored as a string in name.
[sieve] remove dependence on actionflags parameter

The imap4flags extension introduced an actionflags parameter.  It
helped decide whether to use the internal imapflags variable when
appending a message, or whether to use user-specified flags when filing
the message.  This commit removes dependence on that parameter.  The
actionflags parameter can now safely be removed.