Add/use new function findstr_list() which opens and returns a string list

suitable for passing to findstr_in_list().
SBBSecho peformance improvement: don't open/read the twitlist.cfg file for
*each* imported message: just read it once during initialization (using the new
findstr_list() function of course).
Reversed course on the findstr()/trashcan() matching logic: significant leading
white-space was not backwards compatible (and was the cause of recent lost
messages in DOVE-Net) - so I decided to go a different route and support
C-style character escape sequences (e.g. \r, \n, \t, \x##, etc.) in findstr
comparison strings, so the new way to represent a leading space character
in a filter file (e.g. twitlist.cfg, name.can) would be: "\ ". So to match (e.g.
filter/disallow) all strings with a leading space: "\ *". "\x20 *" would also
work (0x20 is ASCII for "space").
Now, again, leading white-space in filter files (e.g. text/*.can, twitlist.cfg)
is ignored. <sigh>