This simply replaces 'ident ( args )' with  'expr ( args )' in the
syntax. A few more changes in the code generator and we're done.

This is in preparation of supporting indirect calls which cause foreign
functions to be reported as undefined variables. Without -W (which is
the default) *no* "undefined variable" warnings will be given. This is
ok, since if they really are undefined the C compiler will flag them as
errors anyway.

This makes foreign declarations obsolete. With hindsight introducing
them was a mistake. I should probably deprecate them.

We no longer include the syntax summary generated from the parser input
file in the reference. This was unreadable anyway. Instead, check that
the syntax productions in the reference are consistent with the
generated ones and complain during build if that is not the case.

Along the way, renamed the generated file from bnf.inc to Syntax.txt and
install it into the html install location; also renamed about.inc to
About.txt, adding both to unused_docs in the sphinx configuration.

Instead of using per channel semaphores to check for completion, the
new implementation uses the request pointer. A NULL pointer means that
no request is pending, i.e. either the request was cancelled or it
completed sucessfully or we lost connection to the PV. The CA callback
resets the request pointer before signalling to the state set where the
request originated, that /some/ event happened. This re-uses the the
existing sync semaphore. ASYNC operations no longer use a semaphore at
all, only the SYNC variants do. However, it becomes even more important
to check whether a disconnect happened, since this is "some event", too,
i.e. it causes pending requests to be cancelled.

The standard EPICS severities (NO_ALARM, MINOR, MAJOR, INVALID) should be
reserved for the actual severity of the underlying PV which we get via CA.

I forgot to add the array index to the varId when iterating
over a multi-PV array.

This patch also gets rid of the ugly seq_sync_timeout hack.