ps: new wchanf option not to strip wchan raw data for clarity

A user reported a situation that stripping do_ and sys_ from raw wchan data caused confusion among other wait channels containing these prepositions.
E.g. (citing):

ps was showing "get_write_access", that is a generic function in the VFS layer, that locks an inode, while it actually was in "do_get_write_access", that is a jbd2 function

Resolves Red Hat Bugzilla #1322111

This patch adds new 'ps' format option wchanf, that shows unstripped wchan data.