Fix directory read serialization bottleneck

The directory walker used dirreadall call for loading directory
entries into memory before assigning them to workers -- causing
severe and unwanted serialization that was especially noticeable
on long directories.

The solution is simple: chunk up the directory read and assign
work in between the chunks.
cool uris don't change
mkfile: *actually* install to /$objtype/bin; derp (thanks qwx)
mk install to the system path (alas)
fix manpage install target
update manual page SOURCE section
take it, it's yours
mkfile: install to $home/bin, as is Correct
ricing the code
fix double-free
implement -T flag that disables the safe-mode
simplify nblk calculation
fix not failing on error
implement robust cloud-scale format-agnostic parallel asynchronous ghost-copy data replication strategy
fix troff references
correctly handle partial reads
fix the write error check
remove stupid mkfile rules. (thanks BurnZeZ)
nicer exit on usage error. (thanks BurnZeZ)