Flymake's flymake-proc.el backend slightly easier to debug

Flymake's flymake-proc.el backend slightly easier to debug

Misc cleanup in flymake-proc.el

Improve description of what this file contains.

Better name for the backend function.  Fix the case where it is run
interactively.

Keep the output buffer alive iff the external process panics.

* lisp/progmodes/flymake-proc.el
(flymake-proc-legacy-flymake): Rename from
flymake-proc-start-syntax-check.  Allow running interactively.
(flymake-start-syntax-check): Obsolete alias for
flymake-proc-legacy-flymake.
(flymake-proc-start-syntax-check): Delete.
(flymake-diagnostic-functions): Include flymake-proc-legacy-flymake
(flymake-proc--process-sentinel): Keep output buffer alive.
Clarify with comments.
(flymake-proc--diagnostics-for-pattern)
(flymake-proc--process-sentinel)
(flymake-proc--safe-delete-directory)
(flymake-proc--start-syntax-check-process): Use condition-case-unless-debug.