daemon: Fix parse_btrfsvol to use STRPREFIX instead of bogus strncmp.

This fixes commit 47b929b7893b3a76ff22760d245cb80720ae6345.
daemon: parse_btrfsvol: Move variable decls to top, whitespace fixes.

This fixes commit 47b929b7893b3a76ff22760d245cb80720ae6345.