[Xastir] OSM tile handling improvements

[Jerry Dunmire]

I just committed changes to CVS to improve the OSM tile download and
cache. These are effective only if libcurl support is enabled:

 - check for tile updates for tiles that have been cached for more
than 7 days (per OSM tile usage policy).
 - use http pipelining (may not help much, since it appears that OSM
servers do not support pipelining for tile downloads.)
 - new debug level, 8192, for verbose output from libcurl

73,
...jerry
KA6HLD