(reftex-finding-files): New customize group.

(reftex-finding-files): New customize group.
(reftex-texpath-environment-variables,
reftex-use-external-file-finders, reftex-external-file-finders,
reftex-search-unrecursed-path-first): New options.
(reftex-process-string, reftex-find-file-externally): New
functions.
(reftex-access-search-path): Execute shell commands to find the
search path.
(reftex-toggle-auto-view-crossref): When turning on, make sure
`reftex-auto-view-crossref' becomes non-nil.
(reftex-abbrev-regexp): Constant removed.
(reftex-convert-string): Abbrev regexp calculated locally.