update taskqueue(9) manual page

Many thanks to bjk, gjb and pluknet for improvements and suggestions.

MFC after:	5 days
Sponsored by:	HybridCluster
provide fast versions of ffsl and flsl for i386; ffsll and flsll for amd64

Reviewed by:	jhb
MFC after:	10 days
X-MFC note:	consider thirdparty modules depending on these symbols
Sponsored by:	HybridCluster
zpool.8: fix typo in option description of labelclear command

MFC after:	5 days