More accurate scheduling of view updates

Don't loose view update requests.  It's not that no requests will be
lost now, but this will happen **much** rarely.
Merge branch 'more-accurate-view-updates'

More accurate scheduling of view updates.
Don't ignore errors, prevent them
Don't generate tags twice
Document updates
Generate tags with helpztags if perl is available
Fix Vim documentation tags with bars (|)
Simplify tags.c generation command

It wasn't properly updated on update.
Use "which" instead of "whereis" in Makefiles

Have no idea, why it was "whereis", which is wrong.
Don't hardcode path to perl
Invoke Vim on build in a more portable way
Make setting filetype=vifm in INSTALL less likely
Note optional build time dependency on perl/vim
Merge branch 'update-tags-generation'

Updated how tags are generated on build.

Thanks to Hendrik Jaeger (a.k.a. henk).