only use fs-legal chars in names (closes #1010);

uploading a folder named COMPLE:X into exfat on linux would fail
because exfat behaves like windows, rejecting <>:|?*"\/

this would also fail on windows, but then due to
sanitize_fn being overly aggressive

fix this by detecting filesystem traits on startup and
also translating vpath early on windows

tests/util.py

@@ -193,6 +193,7 @@ class Cfg(Namespace):
             du_who="all",
             dk_salt="b" * 16,
             fk_salt="a" * 16,
+            fsnt="lin",
             grp_all="acct",
             idp_gsep=re.compile("[|:;+,]"),
             iobuf=256 * 1024,