Add new quicksearch for header's content only

• The new “H S” filter searches for mail that contains string S in
  the content of any non-internal header.

Based on patches¹² by H.Merijn Brand <h.m.brand@xs4all.nl>, with some
reversion, refactoring, minor improvements and fixed typos.

¹ http://lists.claws-mail.org/pipermail/devel/2014-August/001257.html
² http://lists.claws-mail.org/pipermail/devel/2014-August/001258.html
fix bug 3265, 'procmime.c: unbalanced flockfile() / funlockfile()'

Patch by Timo Teräs
Add forgotten author of @^^ patch (and others)
RSSyl: Do not use g_strsplit_set() to get the ID from message file, since the ID itself can contain < or > characters. Should fix bug #3282.