Compile static libexec binaries for gotweb, keeping them separate from the main build. Use the same source files but a separate set of Makefiles. Previously, 'make web-install' installed whichever libexec binaries had already been built, and if those happened to be dynamically linked they failed to run inside the chroot which resulted in "privsep peer process closed pipe" being rendered on the index page.

The command could fail with "got: no such entry found in tree". This problem is reproduced by the regression test added in this commit. This happened because file index entries were processed in the wrong order by diff_fileindex_dir(). To fix this, keep removed entries in the RB tree and skip them when the file index is written out, rather than removing entries from the RB tree immediately causing side-effects for RB_NEXT and friends.

The problem occured inside /usr/src/sys due to the /sys symlink, where /usr/src was a got work tree. If a work tree is present we already have the right path so checking the disk is pointless.