Make checkstyle aware of message priorities.

Known bug: this breaks if the name of the file being checked contain one
of the characters <, >, ', ", &.
Shell escaping is safer since commit a1e1108.
Replace lmake with equivalent system + lgetexpr.

It's now possible to deal with \0 characters in checkers' output.
Shell escaping is saner and safer.
SyntasticMake() has a new option 'preprocess'.
Checkers html/validator and eruby/ruby now use the new preprocess
option.  Auxilliary script validator_decode.awk is no longer needed.
Validator checker: check curl return status.
Undo most of the changes to eruby/ruby.

The new proprocess option is not useful here.
Merge branch 'fix_checkstyle' into preprocess
Fix checkstyle error levels. Closes #709.
Fix shell escaping.
Fix shellpipe expansion.
Fix shellpipe expansion again.
Fix command output redirection.  Cleanup.
Typo.
Give up trying to use readfile().
Add option "syntastic_aggregate_errors".
Merge branch 'master' into preprocess
Bug fix: wrong variable scope.
Bug fix: wrong variable scope.
Catch up with the latest pylint contortions.
Merge branch 'master' into preprocess
Reset only the LC_MESSAGES part of locales.