gvl: Remove focusDistance()

It's consuming a lot of CPU when having lists with thousand entries and it's
not really used for anything
linux: Detect and use valgrind.h
Optimize dictcmp() quite a bit
Use a redblack tree for indexing directory items
httpclient: Fix a regression when using range queries to determine filesize

Caused problems with minidlna (possibly other servers)

Fixes #1576